Trifluoroacetic acid in biological materials has been quantitatively determined by gas chromatography. Benzyl trifluoroacetate has been prepared by the reaction of the acid with phenyldiazomethane, and has been successfully analyzed by gas chromatography without any interference from other peaks. The procedure has been used to determined trifluoroacetic acid in microsomal suspension incubated with halothane, a gaseous anaesthetic.