Thermal degradation of polystyrene (PS) involving brominated polyepoxy (BA) and antimony oxide (PS/BE/Sb2O3) was studied systematically via direct pyrolysis mass spectrometry. Thermal decomposition of brominated polyepoxy was started by loss of end groups. The relative yields of high mass thermal degradation products of PS and the product distribution of brominated polyepoxy and antimony oxide were changed noticeably during the pyrolysis of PS/BE/Sb2O3 composite. Its thermal decomposition was initiated by the interactions of Sb2O3 with BE, and shifted to lower temperatures. Interactions of flame retardants with PS caused degradation of PS at lower temperatures to a certain extent. Loss of HBr and H2O, in the temperature region where main PS decomposition took place, confirmed chemical interactions between the flame retardants and PS. (C) 2013 Elsevier B.V. All rights reserved.