A new sulfur-containing photosensitive monomer, S-methacryloyl O-ethyl xanthate (MAX), was synthesized and characterized. Attempts to make the corresponding S-acryloyl O-ethyl xanthate (AX) under identical experimental conditions resulted in the formation of S-(3-hydroxypropionyl) O-ethyl xanthate as the major product. The free-radical copolymerization behaviour of MAX with methyl methacrylate (MMA) and styrene (St) has been investigated in detail. The copolymer yield and molecular weight were found to decrease with increasing MAX concentration in the monomer feed, which is attributed to the high chain transfer property of MAX. Copolymer compositions determined independently by sulfur and H-1 n.m.r. analyses were in good agreement. Reactivity ratios of the monomers were calculated by Finemann-Ross, Kelen-Tudos and non-linear least-squares methods. Determination of Q and e values reveals that MAX is an electron-deficient monomer and is more reactive towards electron-rich monomers such as styrene. The MAX-St monomer system has a greater tendency towards alternation during copolymerization than the MAX-MMA monomer system. The thermal stability of the copolymers was found to decrease with increase in MAX mole fraction.