Synthetic triolein and tripetroselinin mixtures were examined by13C NMR spectroscopy, showing marked chemical shift differences of the olefinic carbon atoms. Peak height ratios were compared to weight values for quantitative determination of oleic and petroselinic acids in seed oils, since these two fatty acids are quantitated together by GC analysis. Values observed for NMR peak height ratios were fairly close and agreed well with weight ratios. From overall compositions of eleic and petroselinic acids obtained by GC and relative compositions given by13C NMR, petroselinic acid has been determined in tenUmbellifloraeseed oils.