The thermal behaviours of sulphur, selenium and their mixtures have been studied over the range 40–450‡. It has been shown that the polymerization threshold temperature of sulphur,TØ, decreases with increasing selenium content and follows the equilibrium copolymerization model proposed by Tobolsky and Owen. The formation of octa-atomic species Se8−xSx, where 8 >x > 4, takes place only after sulphur is in the liquid state. The rate of polymerization is enhanced by the addition of increasing amounts of selenium and this is reflected in the higher polymerization peak temperatures. The X-ray powder diffractograms show that all the sulphur-selenium melts belong to the same phase as that of SeS, though the constituent atoms are randomly distributed.