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Trans fat content is determined by infrared absorbance at 966 cm.–1 For high accuracy, common interfering absorptions associated with the glycerol backbone of triglycerides and the carboxyl group of fatty acids must be eliminated by conversion of these test samples to their methyl esters prior to analysis. This method is applicable to the accurate determination of isolated trans bonds in natural or processed long chain acids, esters, and triglycerides with trans levels =5%. It is not applicable or is applicable only with specific precautions for fats and oils containing large quantities (over 5%) of conjugated unsaturation, such as tung oil; to materials containing functional groups which modify the intensity of the C-H deformation about the trans double bond, such as castor oil containing ricinoleic acid or its geometrical isomer, ricinelaidic acid (12-hydroxy-9-octadecenoic acid); to mixed glycerides having long- and short-chain moieties, such as diacetostearin; or in general, to any materials containing constituents which have functional groups which give rise to specific absorption bands at or sufficiently close to interfere with the 966 cm–1 band of the C-H deformation of the isolated trans double bond. For accurate determinations on materials with trans levels below 5%, AOCS Method Ce 1h-05 is recommended. For the direct analysis of triglycerides, Association of Official Analytical Chemists (AOAC) method 965.35 is recommended. Replaces Cd 14-61.