Butyl and halogenated butyls are clearly indicated by both DSC and DTG in nitrogen. Binary NR/CIIR blends are also easily characterized by combined DSC and DTG techniques. In ternary SBR/CIIR/NR blends, only CIIR and NR show up in DTG curves when SBR occurs as a minor component. Minor indication of SBR is obtained from DSC curves. Thermal methods fail to distinguish between BUR and CIIR. In elastomer blends, DTG peak temperatures may vary over a wide range depending on the type of the second elastomer. The NR peak temperature (365°C), for example, is lowered by blending with SBR, BR (345–350°C) or CR (325–348°C). These latter elastomers degrade by exothermic reaction. An increase in peak temperature was observed in association with the polymers which degrade by endothermic reaction (EPDM, 369–374°C; CIIR, 373–377°C). This may be caused by abstraction of heat by the second elastomer in the overall decomposition process. Another important conclusion drawn from the blends studies in this series is that thermal stability of the polymers is not materially affected by the presence of another polymer, thus giving rise to the weight-loss peaks at well-defined temperatures, characteristic of each polymer. Thermooxidative stability, however, is very much dependent on the type and composition of the polymers present. The effect of secondary reactions on the degradation curves is also considerably greater in the oxidizing atmosphere. Thermooxidative degradation is very much dependent on sample size, characteristic of a diffusion-controlled reaction. However, it provides auxiliary evidence for idsntification. Except for SBR/BR blends, which show a single Tg changing with composition, all other elastomer blends show transitions at the respective temperatures, indicating the inherent incompatibility of elastomers. However, a second or third transition is not observed when (a) the proportion of the polymer is too small and (b) Tg'a are too close to each other. Thus, obtaining a single glass transition temperature in a polymer blend is not always a proof of its compatibility, as has been claimed so often.