The authenticity of single seed vegetable oils which utilise the C3 photosynthetic pathway was investigated using gas chromatography-combustion-stable isotope ratio mass spectrometry (GC-C-SIRMS). Samples of authentic groundnut, palm, rapeseed and sunflower oils were derivatised to form fatty acid methyl esters (FAMEs) and their carbon isotope ratios (13C12C) determined. In-house reference materials (IHRMs) and internal standards were used routinely to monitor the extraction procedure and SIRMS measurement. These materials demonstrated the consistent performance of the technique. δ13C%. data for the authentic vegetable oil fatty acids fell into the narrow range of −27.6%. to −32.1%. However, the values within the oil varieties considered were significantly different. The data from sunflower oils were such that they could be separated from the other varieties by canonical discriminant analysis. The determination of fatty acid carbon isotope ratios may therefore provide an additional indication of the varietal authenticity of oils which use the C3 photosynthetic pathway.