An investigation into the mechanism of formation of poly(1,4-phenylene sulfide) (PPS) by solution polymerization of copper (I) 4-bromobenzenethiolate (CBT) at 200 degrees C in quinoline under nitrogen at atmospheric pressure is reported. The polymerization has been monitored as a function of reaction time by electron spin resonance (e.s.r.) spectroscopy and by evaluating the yield and molar mass of the PPS formed. The e.s.r. spectra show the existence of organic free radicals throughout the polymerization and also reveal the presence of Cu2+ ions during the period, up to about 2h reaction time, when PPS yield and molar mass increase rapidly, suggesting initiation by single-electron transfer (SET) from Cu+. Ultra-violet radiation and 1,3-dinitrobenzene are shown to have significant effects on the yield and molar mass of the PPS, lending further support to the argument for a mechanism involving free radicals. Confirmation that CBT is capable of undergoing irreversible reactions via both reductive and oxidative SET processes is provided by results from cyclic voltammetry of C-BT solutions. The results from the study, therefore, provide strong evidence for operation of a SET mechanism in solution polymerization of CBT.