Further evidence that iso-linoleic acid is related to the unpleasant odour development when partially hydrogenated linseed oil is heated is as follows:(a) Partially hydrogenated perilla oil, containing considerable iso-linoleic acid, developed the same odour when heated, as did partially hydrogenated linseed oil.(b) A concentrate of iso-linoleic acid developed a similar odour when it was heated. Distillation of the acid, or removal of unsaponifiable substances, had no effect on the odour development.(c) When fatty acid fractions obtained by crystallization of the acids from hydrogenated linseed oil were re-esterified with glycerol, and subjected to the heat test, most odour development occurred in the fraction containing most iso-linoleic acid.Increasing the selectivity of hydrogenation did not greatly affect the formation or hydrogenation of iso-linoleic acid. However, products of high temperature (200° to 250 °C.) hydrogenations were softer for equivalent iodine numbers, and had lower melting points than those from low temperature hydrogenations.Hydrogenation of partly polymerized linseed oil yielded a product that when heated did not develop the characteristic odour of hydrogenated linseed oil.