Dimer acids, or dimerized fatty acids, are dicarboxylic acids prepared by dimerizing unsaturated fatty acids obtained from tall oil, usually on clay catalysts. Dimer acid usually contains predominantly a dimer of oleic acid. It is also called C36 dimer acid. Trimer acid is a corresponding material where the resulting molecule consists of three fatty acid molecules. Dimer acids can be converted to dimer amines by reaction with ammonia and subsequent reduction.