diff --git a/lib/oga/xml/document.rb b/lib/oga/xml/document.rb
index ae82630..9a995ee 100644
--- a/lib/oga/xml/document.rb
+++ b/lib/oga/xml/document.rb
@@ -89,6 +89,23 @@ module Oga
         end
       end
 
+      ##
+      # Returns the available namespaces. These namespaces are retrieved from
+      # the first element in the document.
+      #
+      # @see [Oga::XML::Element#available_namespaces]
+      #
+      def available_namespaces
+        children.each do |child|
+          # There's no guarantee that the first node is *always* an element
+          # node.
+          return child.available_namespaces if child.is_a?(Element)
+        end
+
+        # In case the document is empty.
+        return {}
+      end
+
       ##
       # Converts the document and its child nodes to XML.
       #
diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index c2b6bfe..148c79a 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -410,7 +410,11 @@ module Oga
         name  = ast_node.children[1]
 
         context.each do |context_node|
-
+          context_node.available_namespaces.each do |_, namespace|
+            if namespace.name == name or name == '*'
+              nodes << namespace
+            end
+          end
         end
 
         return nodes
diff --git a/spec/oga/xml/document_spec.rb b/spec/oga/xml/document_spec.rb
index 5c68e5c..e475b85 100644
--- a/spec/oga/xml/document_spec.rb
+++ b/spec/oga/xml/document_spec.rb
@@ -71,6 +71,20 @@ describe Oga::XML::Document do
     end
   end
 
+  context '#available_namespaces' do
+    example 'return an empty Hash by default' do
+      described_class.new.available_namespaces.should be_empty
+    end
+
+    example 'return the namespaces of the first element' do
+      namespace = Oga::XML::Namespace.new(:name => 'x')
+      element   = Oga::XML::Element.new(:namespaces => {'x' => namespace})
+      document  = described_class.new(:children => [element])
+
+      document.available_namespaces['x'].should == namespace
+    end
+  end
+
   context '#to_xml' do
     before do
       child = Oga::XML::Comment.new(:text => 'foo')
diff --git a/spec/oga/xpath/evaluator/axes/namespace_spec.rb b/spec/oga/xpath/evaluator/axes/namespace_spec.rb
index 6df1acb..72797fd 100644
--- a/spec/oga/xpath/evaluator/axes/namespace_spec.rb
+++ b/spec/oga/xpath/evaluator/axes/namespace_spec.rb
@@ -12,6 +12,14 @@ describe Oga::XPath::Evaluator do
       @evaluator = described_class.new(@document)
     end
 
+    context 'matching namespaces in the entire document' do
+      before do
+        @set = @evaluator.evaluate('namespace::*')
+      end
+
+      it_behaves_like :empty_node_set
+    end
+
     context 'matching namespaces in the root element' do
       before do
         @set = @evaluator.evaluate('root/namespace::x')
@@ -30,17 +38,17 @@ describe Oga::XPath::Evaluator do
 
     context 'matching namespaces in a nested element' do
       before do
-        @set = @evaluator.evaluate('root/namespace::x')
+        @set = @evaluator.evaluate('root/foo/namespace::*')
       end
 
       it_behaves_like :node_set, :length => 2
 
-      example 'return the "x" namespace' do
-        @set[0].name.should == 'x'
+      example 'return the "y" namespace' do
+        @set[0].name.should == 'y'
       end
 
-      example 'return the "y" namespace' do
-        @set[1].name.should == 'y'
+      example 'return the "x" namespace' do
+        @set[1].name.should == 'x'
       end
     end