The effect of the presence of iron as a catalyst during base oil oxidation has been studied. Fourier transform infrared (FTIR) spectroscopy was combined to Principal Component Analysis (PCA) in order to extract chemical information during oil oxidation process. After 8 h, the presence of iron did not lead to significant change in the global chemical composition as revealed by the first principal component. However, the second principal component indicated clearly that the initial formation of alcohols and esters was favoured by the presence of iron. The influence of iron was highlighted by PCA analysis of online FTIR data. (C) 2009 Elsevier B.V. All rights reserved.