A novel thermogravimetric analysis test method has been developed for the evaluation of lubricants. It involves superimposing the thermogravimetric trace of the sample in argon over the trace produced in oxygen atmosphere. The difference of the two traces indicates the oxidative influence on volatility as well as on the formation tendencies of high boiling (high molecular weight) oxidation products. The method was illustrated with a base oil with and without a catalyst package and an additive package. The test method is potentially useful for lubricant screening, quality control, and base oil composition effects on thermal oxidative stability.