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DEFINITION This method is for the determination of fatty acid composition of partially hydrogenated vegetable oils and animal fats, containing more than 5% trans fatty acids, by a combined capillary gas–liquid chromatography (GLC)–infrared spectrophotometry (IR) procedure. SCOPE This method is a research method and is not practical for use in normal operations, especially QA/QC work. The method will provide accurate values, but requires considerable experience in its application. This method is designed to evaluate, by combining the fatty acid data determined using capillary GLC with a very polar flexible fused silica column, with the total trans content determined by IR, the fatty acid composition, including the percentages of cis- and trans-octadecenoates, of partially hydrogenated oils. The international collaborative study showed that there was no advantage in using the combined GLC–IR method for samples containing < 5% trans fatty acids. Trans-octadecenoate (18:1t), mono-trans-octadecadienoate (18:2ct or tc, written collectively as 18:2t), trans,trans-octadecadienoate (18:2tt) and mono-trans-octadecatrienoate (18:3cct, ctc and tcc, written collectively as 18:3t) are the trans fatty acids normally encountered in hydrogenated vegetable and animal fats. All these contribute to the total trans content. The total trans content determined by IR using methyl elaidate as the external standard is related to the above component trans fatty acids by the following equation. IR trans = % 18:1t + 1.74(% 18:2tt) + 0.84(% 18:2t + 18:3t). Where— 1.74 = correction factor for trans,trans fatty acids; 0.84 = correction factor for mono-trans fatty acids. By GLC, with very polar columns, the various isomers of 18:2tt, 18:2t and 18:3t are resolved with very little interference ; hence, their weight percentages are determined by GLC. IR yields the total trans content and, therefore, the percentage of 18:t is calculated from the above equation. The difference between the total methyl octadecenoate (18:1), which is the sum of all the 18:1 peaks in GLC, and the calculated 18:1t gives the percentage of cis-octadecenoate. The method is applicable to partially hydrogenated vegetable oils and animal fats, and such oils and fats isolated from food products containing more than 5% trans fatty acids. It is not applicable to partially hydrogenated fish oils, which contain large levels of cis and trans isomers of C16, C18, C20 and C22 chain lengths.