Effect of a few oil-soluble, surface-active agents and the oleates of Cr, Ag, Cd, Pb, Cu on the benzoyl peroxide catalysed polymerisation of styrene in solution and that of the soaps of Mn, Zn, Cd, Co, Ni, Al, Fe etc. on the polymerisation of methyl methacrylate and styrene in absence of peroxide catalyst have been studied. in the former case aerosol and Cu oleate retarded, and cetavlon inhibited poly­merisation. Other soaps studied showed only a very slight retarding effect. In the uncatalysed polymerisation of methyl methacrylate Mn oleate inhibited and Zn oleate and Cd oleate retarded polymerisation, all other soaps catalysed it. With styrene all soaps excepting that of manganese, which inhibited polymerisation, showed definite catalytic activity.