Desaturation of twelve labeled positional isomers of trans-18 : 1 acids was investigated using enzymes of liver microsomes from essential-fatty-acid-deficient rats. Oleic acid was used as model for comparison and for optimizing the incubation conditions. Each positional isomer desaturated at a unique rate. The trans-6- and trans-13-octadecenoic acids gave mostly trans-6, cis-9- and cis-9,-trans-13-18 : 2 acids, respectively. The trans-5-isomer gave mostly cis-5, cis-9-18 : 2 acid. The trans-4-isomer gave a mixture of trans-4, cis-9- + cis-4, cis-9-18 : 2, trans-11-isomer gave cis-9, trans-11- + cis-9, cis-11-18 : 2; trans-12-isomer gave cis-9, trans-12- + cis-9, cis-12-18 : 2, and trans-14-isomer have cis-9,-trans-14- + cis-9, cis-14-18 : 2 acid. The trans-8-, trans-9-, and trans-10-isomers were not measurably desaturated. The site of desaturation of the trans-18 : 1 isomer was the 9-position, indicating action of delta 9 desaturase. Thus the isomeric trans-18 : 1 acids present in partially hydrogenated fats can be converted to cis, trans- or trans, cis- and cis, cis-18 : 2 isomers, and trans-18 : 1 isomers in food may have effects upon metabolic control because of the products derived from them.