A new method is described for detecting the adulteration of maize oil based on differences in <$13C values of oils arising through biosynthetic differences in the mode of carbon dioxide fixation in C4 and C3 plants. Fatty acid composition data are obtained on whole vegetable oils (following saponification and methylation) by GC using a capillary column with a polar stationary phase. 513C values for the individual fatty acids, determined by GCC/ IRMS, are then used together with the compositional data to construct theoretical mixing curves. The validity of these mixing curves was verified experimentally. The ability of the method to detect adulteration of maize oil with C3 oils at low concentrations was tested in a "blind" trial. The technique was readily able to detect adulteration