diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index 422a043..66ed881 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -706,21 +706,33 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def on_call_local_name(context, expression = nil)
-        if expression
-          node = process(expression, context)
-
-          if node.is_a?(XML::NodeSet)
-            node = node.first
-          else
-            raise TypeError, 'local-name() only takes node sets as arguments'
-          end
-        else
-          node = current_node
-        end
+        node = context_node(context, expression)
 
         return node.is_a?(XML::Element) ? node.name : ''
       end
 
+      ##
+      # Processes the `namespace-uri()` function call.
+      #
+      # This function call returns the namespace URI of one of the following:
+      #
+      # * The current context node (if any)
+      # * The first node in the supplied node set
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_call_namespace_uri(context, expression = nil)
+        node = context_node(context, expression)
+
+        if node.is_a?(XML::Element) and node.namespace
+          return node.namespace.uri
+        else
+          return ''
+        end
+      end
+
       ##
       # Processes an `(int)` node. This method simply returns the value as a
       # Float.
@@ -744,6 +756,30 @@ module Oga
         return ast_node.children[0]
       end
 
+      ##
+      # Returns the node for a function call. This node is either the first node
+      # in the supplied node set, or the first node in the current context.
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Oga::XML::Node]
+      #
+      def context_node(context, expression = nil)
+        if expression
+          node = process(expression, context)
+
+          if node.is_a?(XML::NodeSet)
+            node = node.first
+          else
+            raise TypeError, 'only node sets can be used as arguments'
+          end
+        else
+          node = current_node
+        end
+
+        return node
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.
diff --git a/spec/oga/xpath/evaluator/calls/namespace_uri_spec.rb b/spec/oga/xpath/evaluator/calls/namespace_uri_spec.rb
new file mode 100644
index 0000000..54e752b
--- /dev/null
+++ b/spec/oga/xpath/evaluator/calls/namespace_uri_spec.rb
@@ -0,0 +1,38 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'namespace-uri() function' do
+    before do
+      @document  = parse('<root xmlns:x="y"><x:a></x:a><b></b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'outside predicates' do
+      example 'return the namespace URI of the <x:a> node' do
+        @evaluator.evaluate('namespace-uri(root/x:a)').should == 'y'
+      end
+
+      example 'return an empty string when there is no namespace URI' do
+        @evaluator.evaluate('namespace-uri(root/b)').should == ''
+      end
+
+      example 'raise TypeError for invalid argument types' do
+        block = lambda { @evaluator.evaluate('namespace-uri("foo")') }
+
+        block.should raise_error(TypeError)
+      end
+    end
+
+    context 'inside predicates' do
+      before do
+        @set = @evaluator.evaluate('root/*[namespace-uri()]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <x:a> node' do
+        @set[0].should == @document.children[0].children[0]
+      end
+    end
+  end
+end