Thermal degradation of blends containing SAN and EPDM was investigated and compared with that of graft EPDM-g-SAN copolymer molecules. Characterization of decomposition products and of degradation residues was carried out in order to reveal the differences in the degradation processes. With plain mixtures of SAN and EPDM, degradation behaviour is practically additive whereas in the case of EPDM-g-SAN lower volatile products and larger chain scissions are produced. The results can be explained by considering that the degradation process is influenced by the dispersion of the blend components. In phase segregated systems very few interactions take place between the different types of polymers, whereas in the graft copolymer molecular dispersion favours reactive interactions between the structurally different components.