The structures of cyclopentenyl fatty acids in the seed oils ofHydnocarpus anthelmintica Pierre et Laness.,H. alphina Wight,H. hainanensis (Merr.) andTaraktogenos merrilliana C.Y. Wu, all belonging to the family Flacourtiaceae, have been identified by GC-MS of the mixed 2-substituted 4,4-dimethyloxazoline (DMOX) derivatives. These variations are characterized by the presence of key fragments at (M-43), (M-55) and (M-67), while the location of a double bond in the chain is indicated by a 12-mass interval between the highest peaks of two adjacent homologous ion clusters. The unique fragmentation pattern coupled with the good gas chromatographic qualities of these derivatives enable convenient structure determination of cyclopentenyl fatty acids in complex mixtures.