The infrared absorption spectrum of methyl isolinoleate, separated from the methyl esters of hydrogenated linseed oil fatty acids, has a well defined absorption band with maximum absorption at about 968 cm.−1 As an identical band was found in the spectra of fatty acids or esters after isomerization with selenium, it is attributed to the presence of double bonds with a trans-configuration. It was found that the same band was present in the spectra of samples taken during hydrogenation of oils, and that its intensity increased to a maximum, then decreased as hydrogenation proceeded. It is concluded that hydrogenation is accompanied by a cis-to-trans change in some of the double bonds of the fatty acids, and that methyl isolinoleate has at least one double bond with a trans-configuration.