The formation of polybrominated dibenzo-p-dioxins and -dibenzofurans (PBDDs and PBDFs) during pyrolysis of two polymer blends containing decabromobiphenyl and tetrabromobisphenol-A, was investigated at different temperatures and carrier gas compositions. During pyrolysis of a polybutylene terephthalate/decabromobiphenyl blend PBDFs are formed at a ppm level. Even when there is oxygen available in the carrier gas, PBDDs are formed at a much lower level than PBDFs. In the presence of 10% oxygen the maximum yield of tetra- to octabromodibenzofurans is 70 ppm at 600-degrees-C. The contribution of 2,3,7,8-tetrabromodibenzofuran and 1,2,3,7,8-pentabromodibenzofuran is less than 4.5 ppm. During pyrolysis of acrylonitrile/butadiene/styrene with tetrabromobisphenol-A mono- to pentabrominated dibenzofurans were formed at a ppb level. The optimum temperature of formation of PBDDs and PBDFs was 600-degrees-C. The thermal degradation processes of these two polymer blends were investigated in a thermogravimetric analysis. Both flame retardants do not exert an influence on the elementary chemical degradation processes of the polymer blends. The flame retardant activity of decabromobiphenyl and tetrabromobisphenol-A consists of the emission of brominated species into the gas phase which scavenge the propagation radicals and reduce inflammability. The mechanism of formation of PBDDs and PBDFs from decabromobiphenyl in polybutyleneterephthalate consists of a combination of a condensed phase and a gas phase mechanism. In the case of tetrabromobisphenol-A only a gas phase mechanism of formation of PBDDs and PBDFs was observed.