The official regulations for the control of the olive and olive pomace oils of the European Union (EU) and International Olive Oil Council (IOOC) include the determination of fatty acids in order to be applied to several purity criteria. The determination of fatty acids require the preparation of the fatty acid methyl esters (FAME) for the subsequent analysis by gas chromatography with good precision and reproducibility. Among the methods used in the laboratories of both the industries and the official institutions looking after the olive oil control, the ones selected were: 1) cold methylation with methanolic potash and 2) hot methylation with sodium methylate followed by acidification with sulphuric acid in methanol and heating. A statistical assessment of the precision characteristics were performed on the determination of fatty acids using both methods by a collaborative trial following the directions included in the AOAC regulation (AOAC 1995). In oils with low acidities, the results obtained for both methylation methods were equivalent. However, the olivepomace oil sample (acidity 15.5%) showed significative differences between the fatty acid compositions obtained using both methylation methods. Finally, the methylation with the acidic+basic method did not yield an increase of the trans-isomers of the fatty acids.