Isothermal thermogravimetry has been used to assess the influence of antioxidants on thermooxidative conversions of petroleum and synthetic oils and also certain oligomers. It has been shown that the method makes it possible to test the thermo-oxidative stability of polymers from the induction period of their oxidation, and for synthetic oils from their weight loss. Furthermore, the method can be used in selecting the optimum temperatures for the application and processing of polymers, and also for the effective use of synthetic oils. For petroleum oils there are limitations connected with their high volatility at temperatures above 150-degrees-C.