The effect of the presence of controlled amounts Of O-2 upon microcarbon residue (MCR) determination is presented. The amount Of O-2 leading to an increase in MCR that is mistakenly considered as trace or small has been quantified by performing experiments using a thermogravimetric analyzer (TGA) and heating the samples in a muffle furnace under different O-2 concentrations. For a fixed sample amount, MCR may increase or decrease depending upon the O-2 concentration in the atmosphere above the sample. TGA results indicate that, in addition to pyrolysis that occurs at all times during heating of the sample, there are two competitive reactions, namely, partial oxidation and combustion that dominate each other depending upon the O-2 concentration above the sample. If the O-2 content appears limiting, there would be an increase in MCR because partial oxidation would dominate over the combustion. If the amount of the sample is limiting, then the O-2 concentration would be high enough for the combustion reaction to dominate over partial oxidation, leading to low MCR values. Commercially available MCR standards are studied in this work. Their Fourier transform infrared (FTIR) spectra determined before and after heating in an inert or oxidative atmosphere show that the presence of small amounts Of O-2 affect these samples to varying levels depending upon their chemical composition. The study carried out with few standard samples may not appear appropriate to apply on a broad range of oil samples; however, the achieved results suggest that MCR determinations should better be conducted under truly inert conditions.