Concentrations of trans isomers of 18:2w6 and 18:3w3 were measured in soybean and canola oils purchased in the U. S. Fatty acid methyl esters were prepared by BF3 catalyzed transesterification and separated using gas chromatography with Omegawax and CPSil88 capillary columns. Fatty acids were identified by comparing retention times with standards and with isomers of 18:2w6 and 18:3w3 in elaidinized linseed oil. Isomers identified included 18:2ct, 18:2tc, 18:3tct+ctt, 18:3cct, 18:3ctc, and 18:3tcc. The degree of isomerizations of 18:2w6 and 18:3w3 ranged from 0.3% to 3.3% and 6.6% to 37.1%, respectively. The trans contents were between 0.56% and 4.2% of the total fatty acids. Consumers will obtain isomerized essential fatty acids from vegetable oils currently marketed in the U. S.