AbstractDebromination side‐reactions which occur during the synthesis of poly(arylene sulfide)s from copper(I) 4‐bromothiophenoxidesSystematic name: Copper(I) 4‐bromobenzenethiolates.were investigated using bromobenzene as a model for the end unit in a poly(1,4‐phenylene sulfide) chain. Model reactions were conducted in quinoline, pyridine, and 10:1 (v/v) quinoline/pyridine solution in the presence and absence of copper(I) bromide, the condensation product of polymerisation. Reaction liquors were subjected to GLC and MS analysis and evidence is presented which indicates that a copper(I) initiated debromination reaction occurs yielding monophenylquinolines. A mechanism is proposed for the reaction and critically discussed. The consequences of the reaction at the macromolecular level are also de