Abstract: A process for reducing the residual monomer level of a polymer latex prepared by emulsion polymerization comprises exposing the latex in a separate vessel from that in which the main polymerization has been carried out to conditions under which free radicals are produced by a secondary catalyst added to the latex, the said catalyst being oil soluble or having an oil soluble component. In a preferred embodiment a polymer latex prepared by the emulsion polymerization of styrene alone or a mixture containing styrene having a residual styrene level not exceeding 0.1% based on latex volume may be obtained.