Abstract: Process for the polymerization of vinyl chloride or the copolymerization of vinyl chloride and vinyl acetate at temperatures of less than about 70.degree. C., in the presence of a polymerization initiator which is a blend of an acetyl alkylsulfonyl peroxide with (A) a peroxydicarbonate, (B) a diacyl peroxide and a tertiary-alkyl neoacid peroxyester or (C) a diacyl peroxide and a peroxydicarbonate. .Iadd.