A modified TP 306 test procedure is used to study the high temperature liquid phase oxidation of five base oils and their hydrocarbon type subfractions. The base oils are selected so as to cover a wide range of physiochemical characteristics. Qualitative as well as quantitative analysis of the base oils and their fractions were done using chromatographic (column, HPLC) and spectroscopic (MS, NMR, FTIR) techniques to gain a better understanding of their compositional and structural details. Oxidized materials were also studied and structural changes identified as a result of thermal oxidation. Relative oxidative degradation of different base oils is discussed in terms of their NMR derived structural models. The results indicate that polyaromatic and naphthenoaromatic compounds undergo more oxidative degradation than saturated molecules. The oxygenated polar compounds and deposits formed are mainly derived from aromatic structures, which decreases as weight percent of the total base fluid as oxidation progresses. Alkyl substituted aromatic structures showed larger decrease in chain length than purely paraffin (n- and iso-) compounds during oxidation. Spectroscopic data also indicated that alkylation of aromatics is mainly through alkyl radicals generated from long chain alkyl aromatics.