A new method is described for distinguishing between animal fats preserved in ancient pottery. Analysis of lipid fractions from two morphologically distinct vessel types (lamps and “dripping dishes”) using on-line gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS) showed that they could be distinguished by plotting the δ¹³C value for n-hexadecanoic acid against that for n-octadecanoic acid. The δ¹³C values obtained for modern reference fats from domesticated animals likely to have been important in antiquity showed the lamp extracts to correlate with ruminant animal fat, such as sheep or cattle, whereas the “dripping dishes” had δ¹³C values similar to those of non-ruminant animal fat, such as pig. These findings were entirely consistent with distributional information obtained by GC and with positional isomer information gained from analysis of dimethyldisulphide derivatives of the monounsaturated fatty acids. The results indicate that GC–C–IRMS has considerable potential for the classification of animal fats absorbed in ancient pottery particularly where fatty acid distributions have been altered by degradation during vessel use or burial. δ¹³C values were also shown to be of value in detecting the use of vessels in the processing of animal products from more than one source.