Dimer acids, or dimerized fatty acids, are dicarboxylic acids prepared by dimerizing unsaturated fatty acids obtained from tall oil, usually on clay catalysts. The CAS number of the material is [61788-89-4]. Dimer acids are used primarily for synthesis of polyamide resins and polyamide hot melt adhesives. They are also used in alkyd resins, adhesives, surfactants, as fuel oil additives, lubricants, etc.^[1] It is a light yellow or yellow viscous transparent liquid.

Dimer acid usually contains predominantly a dimer of oleic acid. It is also called C36 dimer acid.^[2]

Trimer acid is a corresponding material where the resulting molecule consists of three fatty acid molecules. Its CAS number is [68937-90-6].

Dimer acids can be converted to dimer amines by reaction with ammonia and subsequent reduction.

Dimer fatty acids are produced from different fatty acids by heating. Necessary are a fatty acid with conjugated double bonds or other unsaturated fatty acids. Examples of such fatty acids are conjugated linoleic acids. The reaction is carried out via Diels-Alder addition, whereby a partially unsaturated C6 ring is formed.^[3] Besides the dimer, trimers as well as (unreacted) monomers of the fatty acids may be present in the mixture.