The intensity of the thermal oxidation has been followed through kinetic parameters. In styrene/butadiene copolymers, soft polybutadiene phase (PB) and hard polystyrene phase (PS) have different sensibilities to thermal degradation. The thermal oxidative stability of high-impact polystyrene (PS-HI), styrene/butadiene/styrene block copolymers (SBS), and PS-HI + SBS blends were investigated by isothermal DSC analysis. The kinetic parameters of the thermal oxidative degradative process: the reaction rate constants, the reaction orders and the activation energies have been determined. The total area under the exothermic curve was used to calculate the isothermal heats of degradation. The conversion (alpha) and the rate of the thermal oxidative degradation (d alpha/dt) have been derivated. The experimental data have been tested with a kinetic model for autocatalytic reactions: d alpha/dt = k alpha(m) (1 - alpha)(n) Activation energies have been determined from the Arrhenius equation and the order of the thermal oxidative stability evaluated. The extent of agreement of the experimental data with the autocatalytic model is discussed.