diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index ef5c17d..9892610 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -509,6 +509,20 @@ module Oga
         return nodes
       end
 
+      ##
+      # Processes the pipe (`|`) operator. This operator creates a union of two
+      # sets.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_pipe(ast_node, context)
+        left, right = *ast_node
+
+        return process(left, context) + process(right, context)
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.
diff --git a/spec/oga/xpath/evaluator/operators/pipe_spec.rb b/spec/oga/xpath/evaluator/operators/pipe_spec.rb
new file mode 100644
index 0000000..1c886fd
--- /dev/null
+++ b/spec/oga/xpath/evaluator/operators/pipe_spec.rb
@@ -0,0 +1,62 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'pipe operator' do
+    before do
+      @document  = parse('<root><a></a><b></b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'merge two sets' do
+      before do
+        @set = @evaluator.evaluate('/root/a | /root/b')
+      end
+
+      it_behaves_like :node_set, :length => 2
+
+      example 'include the <a> node' do
+        @set[0].name.should == 'a'
+      end
+
+      example 'include the <b> node' do
+        @set[1].name.should == 'b'
+      end
+    end
+
+    context 'merge two sets when the left hand side is empty' do
+      before do
+        @set = @evaluator.evaluate('foo | /root/b')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'include the <b> node' do
+        @set[0].name.should == 'b'
+      end
+    end
+
+    context 'merge two sets when the right hand side is empty' do
+      before do
+        @set = @evaluator.evaluate('/root/a | foo')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'include the <a> node' do
+        @set[0].name.should == 'a'
+      end
+    end
+
+    context 'merge two identical sets' do
+      before do
+        @set = @evaluator.evaluate('/root/a | /root/a')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'include only a single <a> node' do
+        @set[0].name.should == 'a'
+      end
+    end
+  end
+end