diff --git a/lib/oga/css/lexer.rl b/lib/oga/css/lexer.rl
index fc8c50d..dde2b7e 100644
--- a/lib/oga/css/lexer.rl
+++ b/lib/oga/css/lexer.rl
@@ -230,6 +230,8 @@ module Oga
         pseudo_args := |*
           whitespace;
 
+          hash | dot | colon;
+
           # NOTE: the priorities here are put in place to ensure that rules such
           # as  `nth` take precedence over `identifier`. The highest number has
           # the highest priority.
diff --git a/lib/oga/css/parser.rll b/lib/oga/css/parser.rll
index ccffba8..bf978fd 100644
--- a/lib/oga/css/parser.rll
+++ b/lib/oga/css/parser.rll
@@ -497,6 +497,24 @@ even
     s(:call, 'not', s(:axis, 'child', s(:type_test, 'node')))
   end
 
+  # Generates the AST for the `:not` selector.
+  #
+  # @param [AST::Node] arg
+  # @return [AST::Node]
+  def on_pseudo_class_not(arg)
+    # Unpacks (axis "descendant" (test nil "x")) into just (test nil "x") as
+    # in this case we want to wrap the (test) node in a (axis "self") node.
+    if arg.type == :axis
+      arg = s(:axis, 'self', arg.children[1])
+
+    # Unpack (predicate (eq ...)) into just (eq)
+    elsif arg.type == :predicate
+      arg = arg.children[1]
+    end
+
+    s(:call, 'not', arg)
+  end
+
   # Generates the AST for the `=` operator.
   #
   # @param [AST::Node] attr
diff --git a/spec/oga/css/lexer/pseudo_classes_spec.rb b/spec/oga/css/lexer/pseudo_classes_spec.rb
index fac944d..c307fe0 100644
--- a/spec/oga/css/lexer/pseudo_classes_spec.rb
+++ b/spec/oga/css/lexer/pseudo_classes_spec.rb
@@ -126,5 +126,16 @@ describe Oga::CSS::Lexer do
         [:T_RPAREN, nil]
       ]
     end
+
+    it 'lexes the :not(#foo) pseudo class' do
+      lex_css(':not(#foo)').should == [
+        [:T_COLON, nil],
+        [:T_IDENT, 'not'],
+        [:T_LPAREN, nil],
+        [:T_HASH, nil],
+        [:T_IDENT, 'foo'],
+        [:T_RPAREN, nil]
+      ]
+    end
   end
 end
diff --git a/spec/oga/css/parser/pseudo_classes/not_spec.rb b/spec/oga/css/parser/pseudo_classes/not_spec.rb
new file mode 100644
index 0000000..79871fb
--- /dev/null
+++ b/spec/oga/css/parser/pseudo_classes/not_spec.rb
@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe Oga::CSS::Parser do
+  describe ':not pseudo class' do
+    it 'parses the :not(x) pseudo class' do
+      parse_css(':not(x)').should == parse_xpath('descendant::*[not(self::x)]')
+    end
+
+    it 'parses the x:not(y) pseudo class' do
+      parse_css('x:not(y)').should == parse_xpath('descendant::x[not(self::y)]')
+    end
+
+    it 'parses the x:not(#foo) pseudo class' do
+      parse_css('x:not(#foo)').should ==
+        parse_xpath('descendant::x[not(@id="foo")]')
+    end
+  end
+end