The degradation of bisphenol A polycarbonate (PC), poly(methyl methacrylate) (PMMA) and a 1:1 by weight blend of PC and PMMA have been studied by thermogravimetry, thermal volatilisation analysis and differential scanning calorimetry. Volatile products have been investigated and separated by subambient TVA and characterised spectroscopically. In the degradation of the blend, no change is observed in the nature of the volatile products of degradation, but the rate of degradation of the PC component is increased and the PMMA depolymerisation is retarded. It is suggested that PMMA radicals attack PC macromolecules leading to chain scission in the PC at lower temperatures than required for homolysis in pure PC. This unusual form of interaction involving a macroradical and a macromolecule is made possible by the homogeneous character of the molten blend.