Copoly(p-phenylene/biphenylene sulfide)s (PPBS) were synthesized from sodium sulfide and p-dichlorobenzene (DCB)/4,4’-dibromobiphenyl (DBB) comonomers in N-methyl-2-pyrrolidinone (NMP) solvent under high temperature (270 degrees C) and pressure. The copolymer composition and molecular weight of PPBS were determined by elemental(C,H,S) analysis and high temperature (210 degrees C) g.p.c, with 1-chloronaphthalene as eluent, respectively. The reactivity of DBB monomer was found to be higher than that of DCB toward the thiolate anion from the copolymer composition analysis. The molecular weight of PPBS copolymer showed maximum value (M(w) = 23.9-24.1 x 10(3)) at DCB/DBB feed ratio of 70/30-60/40 mol%. The T(g)s of copolymer were in good agreement with Fox’s equation. The PPBS samples exhibited T(m)s over all copolymer compositions by d.s.c. measurement. The eutectic point appeared to be at about 30 mol% of DBB repeat unit in the copolymer.