Drupal 7 Rules: Compare Group Reference Field and fire Rule if data changed
I solved the problem by creating an own condition for rules using the API of the rules module.

function hook_rules_condition_info() {
  return array(
    'rules_condition_text_compare' =&gt; array(
      'label' =&gt; t('Textual comparison'), 
      'parameter' =&gt; array(
        'text1' =&gt; array(
          'label' =&gt; t('Text 1'),
          'type' =&gt; 'text',
        ), 
        'text2' =&gt; array(
          'label' =&gt; t('Text 2'),
          'type' =&gt; 'text',
        ),
      ), 
      'group' =&gt; t('Rules'),
    ),
  );
}


See hook_rules_condition_info for more details.