Adulteration of olive oils can be detected by comparison of the 13C/12C ratios as obtained subsequently by high-resolution gas chromatography (HRGC) and isotope ratio mass spectrometry (IRMS). In δ13C values determined for the aliphatic alcoholic fraction of pomace oil, the adulterant was significantly more negative than those of virgin and refined olive oils. Quantitative analysis of isoprenoids and methylsterols isolated from each grade of olive oil supports the decreasingly negative values of 13C/12C ratios for the better grades of olive oil. These results have turned out to be a good tool for detecting adulteration of olive oil with the cheaper pomace olive oil, even down to a 5% level. Keywords: Carbon stable isotopes; olive oil; adulteration; aliphatic alcohols; waxes