Trans fat content is determined by infrared absorbance at 966 cm–1 using a Fourier transform infrared spectrometer capable of operating at 4 cm-1 resolution, equipped with a TGS or DTGS detector and an attenuated total reflection (ATR) infrared cell. This method requires no weighing and no quantitative dilution of TAG test samples in any solvent. This method is applicable to the rapid (about 5 min) and accurate determination of isolated trans double bonds in natural or processed oils and fats with e levels equal to or greater than about 1.0%. It is not applicable to fats and oils containing over about 1% of conjugated unsaturation, such as tung oil; to materials containing functional groups which modify intensity of the C-H deformation about the trans double bond, such as castor oil, which contains hydroxy groups; or in general, to any materials containing constituents with functional groups that 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 of the materials with trans levels below about 1.0%, gas chromatography of FAME 9e.g., AOCS Method Ce 1h-05) is recommended; GC has also been the industry standard for the routine determination of fatty acid composition. This method requires no weighing and no quantitative dilution of TAG test samples in any solvent.