Copoly(p-phenylene/biphenylene sulfide)s, PPBS were prepared from sodium sulfide trihydate(Na2S . 3H(2)O), p-dichlorobenzene (DCB), and 4,4’-dibromobiphenyl (DBB) comonomers in N-methyl-2-pyrrolidinone (NMP) solvent using an autoclave. The molecular weights of PPBS copolymers were determined by high temperature (210 degrees C) GPC in 1-chloronaphthalene solvent. The reaction temperature had little effect on the molecular weights of PPBS copolymers with water as additive at the level of 3 mol H2O per 1 mol Na2S. PPBS copolymer, however, showed maximum molecular weight of M(w) = 24.1 x 10(3) with the total water content of 9 mol H2O per 1 mol Na2S at an optimum polymerization temperature of 270 degrees C. The resulting PPBS copolymer sample showed higher T-g (by 30 degrees C) and lower T-m (by 10 degrees C) than PPS homopolymer prepared under similar conditions.