The incorporation of both OMPOSS and Exolit OP950 (zinc phosphinate) into PET leads to increased fire retarding properties and a synergistic effect has been established between the three components. Here the thermal degradation of OMPOSS, Exolit OP950, PET and blends of them is investigated via thermal degradation in pyrolytic and thermo-oxidative conditions. All species formed during the degradation of the additives or the blends are identified by solid state NMR and X-ray diffraction in the condensed phase and by CC-MS in the gas phase. The investigation shows that no chemical interaction occurs between the additives, which suggests that the synergy responsible for the improvement of fire properties of the material has a physical origin. (C) 2009 Elsevier B.V. All rights reserved.