diff --git a/.travis.yml b/.travis.yml
index 240f8f2e4..157d8a95a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,6 +14,7 @@ branches:
     - master
 matrix:
   allow_failures:
+    - rvm: rbx-2
     - rvm: ruby-head
     - rvm: jruby-head
     - rvm: 1.9.3
diff --git a/Rakefile b/Rakefile
index 8f76de7be..094554f72 100644
--- a/Rakefile
+++ b/Rakefile
@@ -34,7 +34,7 @@ if defined?(JRUBY_VERSION)
     ext.ext_dir = 'ext'
   end
 
-elsif Concurrent.use_c_extensions?
+elsif Concurrent.allow_c_extensions?
 
   Rake::ExtensionTask.new(EXTENSION_NAME, GEMSPEC) do |ext|
     ext.ext_dir = "ext/#{EXTENSION_NAME}"
diff --git a/ext/concurrent_ruby_ext/extconf.rb b/ext/concurrent_ruby_ext/extconf.rb
index 6ea60adf4..eb635c2f9 100644
--- a/ext/concurrent_ruby_ext/extconf.rb
+++ b/ext/concurrent_ruby_ext/extconf.rb
@@ -12,7 +12,7 @@ def create_dummy_makefile
   end
 end
 
-if defined?(JRUBY_VERSION) || ! Concurrent.use_c_extensions? 
+if defined?(JRUBY_VERSION) || ! Concurrent.allow_c_extensions? 
   create_dummy_makefile
   warn 'C optimizations are not supported on this version of Ruby.'
 else
diff --git a/lib/concurrent/atomic.rb b/lib/concurrent/atomic.rb
index cdd3980d2..93121f0c4 100644
--- a/lib/concurrent/atomic.rb
+++ b/lib/concurrent/atomic.rb
@@ -57,19 +57,16 @@
   class Concurrent::Atomic < Concurrent::JavaAtomic
   end
 
-elsif defined? Concurrent::CAtomic
+elsif defined? Concurrent::RbxAtomic
 
-  # @!macro [attach] concurrent_update_error
-  #
-  # This exception may be thrown by methods that have detected concurrent
-  # modification of an object when such modification is not permissible.
-  class Concurrent::Atomic < Concurrent::CAtomic
+  # @!macro atomic_reference
+  class Concurrent::Atomic < Concurrent::RbxAtomic
   end
 
-elsif defined? Concurrent::RbxAtomic
+elsif Concurrent.allow_c_native_class?('CAtomic')
 
   # @!macro atomic_reference
-  class Concurrent::Atomic < Concurrent::RbxAtomic
+  class Concurrent::Atomic < Concurrent::CAtomic
   end
 
 else
diff --git a/lib/concurrent/atomic/atomic_boolean.rb b/lib/concurrent/atomic/atomic_boolean.rb
index 02e51432b..2e88b2414 100644
--- a/lib/concurrent/atomic/atomic_boolean.rb
+++ b/lib/concurrent/atomic/atomic_boolean.rb
@@ -1,3 +1,6 @@
+require_relative '../../extension_helper'
+Concurrent.safe_require_c_extensions
+
 module Concurrent
 
   # @!macro [attach] atomic_boolean
@@ -159,7 +162,7 @@ def make_false
     class AtomicBoolean < JavaAtomicBoolean
     end
 
-  elsif defined? Concurrent::CAtomicBoolean
+  elsif Concurrent.allow_c_native_class?('CAtomicBoolean')
 
     # @!macro atomic_boolean
     class CAtomicBoolean
diff --git a/lib/concurrent/atomic/atomic_fixnum.rb b/lib/concurrent/atomic/atomic_fixnum.rb
index 3049862bd..a1a2357a4 100644
--- a/lib/concurrent/atomic/atomic_fixnum.rb
+++ b/lib/concurrent/atomic/atomic_fixnum.rb
@@ -1,3 +1,6 @@
+require_relative '../../extension_helper'
+Concurrent.safe_require_c_extensions
+
 module Concurrent
 
   # @!macro [attach] atomic_fixnum
@@ -163,7 +166,7 @@ def compare_and_set(expect, update)
     class AtomicFixnum < JavaAtomicFixnum
     end
 
-  elsif defined? Concurrent::CAtomicFixnum
+  elsif Concurrent.allow_c_native_class?('CAtomicFixnum')
 
     # @!macro atomic_fixnum
     class CAtomicFixnum
diff --git a/lib/concurrent/atomic_reference/jruby.rb b/lib/concurrent/atomic_reference/jruby.rb
index 2958a4640..7533aec01 100644
--- a/lib/concurrent/atomic_reference/jruby.rb
+++ b/lib/concurrent/atomic_reference/jruby.rb
@@ -1,10 +1,14 @@
-require 'concurrent_ruby_ext'
-require 'concurrent/atomic_reference/direct_update'
+require_relative '../../extension_helper'
+Concurrent.safe_require_java_extensions
 
-module Concurrent
+if defined?(Concurrent::JavaAtomic)
+  require 'concurrent/atomic_reference/direct_update'
 
-  # @!macro atomic_reference
-  class JavaAtomic
-    include Concurrent::AtomicDirectUpdate
+  module Concurrent
+
+    # @!macro atomic_reference
+    class JavaAtomic
+      include Concurrent::AtomicDirectUpdate
+    end
   end
 end
diff --git a/lib/concurrent/atomic_reference/ruby.rb b/lib/concurrent/atomic_reference/ruby.rb
index 20d15526f..437e1d3e4 100644
--- a/lib/concurrent/atomic_reference/ruby.rb
+++ b/lib/concurrent/atomic_reference/ruby.rb
@@ -1,8 +1,12 @@
-begin
-  require 'concurrent_ruby_ext'
-rescue LoadError
-  # may be a Windows cross-compiled native gem
-  require "#{RUBY_VERSION[0..2]}/concurrent_ruby_ext"
+require_relative '../../extension_helper'
+
+if Concurrent.allow_c_extensions?
+  begin
+    require 'concurrent_ruby_ext'
+  rescue LoadError
+    # may be a Windows cross-compiled native gem
+    require "#{RUBY_VERSION[0..2]}/concurrent_ruby_ext"
+  end
 end
 
 require 'concurrent/atomic_reference/direct_update'
@@ -14,19 +18,19 @@ module Concurrent
   class CAtomic
     include Concurrent::AtomicDirectUpdate
     include Concurrent::AtomicNumericCompareAndSetWrapper
-    
+
     # @!method initialize
     #   @!macro atomic_reference_method_initialize
-    
+
     # @!method get
     #   @!macro atomic_reference_method_get
-    
+
     # @!method set
     #   @!macro atomic_reference_method_set
-    
+
     # @!method get_and_set
     #   @!macro atomic_reference_method_get_and_set
-    
+
     # @!method _compare_and_set
     #   @!macro atomic_reference_method_compare_and_set
   end
diff --git a/lib/concurrent/version.rb b/lib/concurrent/version.rb
index 60bef231a..505470fda 100644
--- a/lib/concurrent/version.rb
+++ b/lib/concurrent/version.rb
@@ -1,3 +1,3 @@
 module Concurrent
-  VERSION = '0.7.0.rc2'
+  VERSION = '0.7.0.rc3'
 end
diff --git a/lib/extension_helper.rb b/lib/extension_helper.rb
index 4b0ffa7ac..a342b598f 100644
--- a/lib/extension_helper.rb
+++ b/lib/extension_helper.rb
@@ -1,9 +1,28 @@
-require 'rbconfig'
-
 module Concurrent
-  def self.use_c_extensions?
-    host_os = RbConfig::CONFIG['host_os']
-    ruby_name = RbConfig::CONFIG['ruby_install_name']
-    (ruby_name =~ /^ruby$/i || host_os =~ /mswin32/i || host_os =~ /mingw32/i)
+
+  # @!visibility private
+  def self.allow_c_extensions?
+    defined?(RUBY_ENGINE) && RUBY_ENGINE == 'ruby'
+  end
+
+  # @!visibility private
+  def self.allow_c_native_class?(clazz)
+    allow_c_extensions? && Concurrent.const_defined?(clazz)
+  rescue
+    false
+  end
+
+  # @!visibility private
+  def self.safe_require_c_extensions
+    require 'concurrent_ruby_ext' if allow_c_extensions?
+  rescue LoadError
+    #warn 'Attempted to load C extensions on unsupported platform. Continuing with pure-Ruby.'
+  end
+
+  # @!visibility private
+  def self.safe_require_java_extensions
+    require 'concurrent_ruby_ext' if RUBY_PLATFORM == 'java'
+  rescue LoadError
+    #warn 'Attempted to load Java extensions on unsupported platform. Continuing with pure-Ruby.'
   end
 end
diff --git a/spec/concurrent/atomic/atomic_boolean_spec.rb b/spec/concurrent/atomic/atomic_boolean_spec.rb
index 242d04f86..573a97815 100644
--- a/spec/concurrent/atomic/atomic_boolean_spec.rb
+++ b/spec/concurrent/atomic/atomic_boolean_spec.rb
@@ -158,7 +158,7 @@ module Concurrent
     end
   end
 
-  if RUBY_PLATFORM == 'java'
+  if TestHelpers.jruby?
 
     describe JavaAtomicBoolean do
       it_should_behave_like :atomic_boolean
@@ -166,14 +166,20 @@ module Concurrent
   end
 
   describe AtomicBoolean do
-    if defined? Concurrent::CAtomicBoolean
-      it 'inherits from CAtomicBoolean' do
-        expect(AtomicBoolean.ancestors).to include(CAtomicBoolean)
+    if RUBY_ENGINE != 'ruby'
+      it 'does not load the C extension' do
+        expect(defined?(Concurrent::CAtomicBoolean)).to be_falsey
       end
-    elsif RUBY_PLATFORM == 'java'
+    end
+
+    if TestHelpers.jruby?
       it 'inherits from JavaAtomicBoolean' do
         expect(AtomicBoolean.ancestors).to include(JavaAtomicBoolean)
       end
+    elsif defined? Concurrent::CAtomicBoolean
+      it 'inherits from CAtomicBoolean' do
+        expect(AtomicBoolean.ancestors).to include(CAtomicBoolean)
+      end
     else
       it 'inherits from MutexAtomicBoolean' do
         expect(AtomicBoolean.ancestors).to include(MutexAtomicBoolean)
diff --git a/spec/concurrent/atomic/atomic_fixnum_spec.rb b/spec/concurrent/atomic/atomic_fixnum_spec.rb
index 51402046e..ae4e7e832 100644
--- a/spec/concurrent/atomic/atomic_fixnum_spec.rb
+++ b/spec/concurrent/atomic/atomic_fixnum_spec.rb
@@ -172,7 +172,7 @@ module Concurrent
     end
   end
 
-  if RUBY_PLATFORM == 'java'
+  if TestHelpers.jruby?
 
     describe JavaAtomicFixnum do
       it_should_behave_like :atomic_fixnum
@@ -180,14 +180,20 @@ module Concurrent
   end
 
   describe AtomicFixnum do
-    if defined? Concurrent::CAtomicFixnum
-      it 'inherits from CAtomicFixnum' do
-        expect(AtomicFixnum.ancestors).to include(CAtomicFixnum)
+    if RUBY_ENGINE != 'ruby'
+      it 'does not load the C extension' do
+        expect(defined?(Concurrent::CAtomicFixnum)).to be_falsey
       end
-    elsif RUBY_PLATFORM == 'java'
+    end
+
+    if TestHelpers.jruby?
       it 'inherits from JavaAtomicFixnum' do
         expect(AtomicFixnum.ancestors).to include(JavaAtomicFixnum)
       end
+    elsif defined? Concurrent::CAtomicFixnum
+      it 'inherits from CAtomicFixnum' do
+        expect(AtomicFixnum.ancestors).to include(CAtomicFixnum)
+      end
     else
       it 'inherits from MutexAtomicFixnum' do
         expect(AtomicFixnum.ancestors).to include(MutexAtomicFixnum)
diff --git a/spec/concurrent/atomic_spec.rb b/spec/concurrent/atomic_spec.rb
index c98a49ecd..f7343a9b9 100644
--- a/spec/concurrent/atomic_spec.rb
+++ b/spec/concurrent/atomic_spec.rb
@@ -152,14 +152,14 @@ module Concurrent
   end
 
   describe Atomic do
-    if TestHelpers.use_c_extensions?
-      it 'inherits from CAtomic' do
-        expect(Atomic.ancestors).to include(CAtomic)
-      end
-    elsif TestHelpers.jruby?
+    if TestHelpers.jruby?
       it 'inherits from JavaAtomic' do
         expect(Atomic.ancestors).to include(JavaAtomic)
       end
+    elsif TestHelpers.use_c_extensions?
+      it 'inherits from CAtomic' do
+        expect(Atomic.ancestors).to include(CAtomic)
+      end
     elsif TestHelpers.rbx?
       it 'inherits from RbxAtomic' do
         expect(Atomic.ancestors).to include(RbxAtomic)
diff --git a/spec/support/example_group_extensions.rb b/spec/support/example_group_extensions.rb
index d27b2bd45..c99b64eb9 100644
--- a/spec/support/example_group_extensions.rb
+++ b/spec/support/example_group_extensions.rb
@@ -12,19 +12,19 @@ def delta(v1, v2)
     end
 
     def mri?
-      RbConfig::CONFIG['ruby_install_name']=~ /^ruby$/i
+      RUBY_ENGINE == 'ruby'
     end
 
     def jruby?
-      RbConfig::CONFIG['ruby_install_name']=~ /^jruby$/i
+      RUBY_ENGINE == 'jruby'
     end
 
     def rbx?
-      RbConfig::CONFIG['ruby_install_name']=~ /^rbx$/i
+      RUBY_ENGINE == 'rbx'
     end
 
     def use_c_extensions?
-      Concurrent.use_c_extensions? # from extension_helper.rb
+      Concurrent.allow_c_extensions? # from extension_helper.rb
     end
 
     def do_no_reset!