Abstract: A process is disclosed for the preparation of polymers and copolymers of yl choride, which comprises polymerizing vinyl chloride or copolymerizing vinyl chloride and monomers copolymerizable therewith in the presence of a peroxide having the general formula ##SPC1##Wherein R represents an alkyl- or an alkoxy-group having from 1-6 carbon atoms as an initiator. The initiator is used in an amount of 0.01 to 1.0% by weight, calculated on the monomer or monomer mixture. The foregoing peroxide may be used in combination with another structurally similar peroxide or with other peroxides also capable of polymerizing or copolymerizing vinyl chloride in an amount of 0.01 to 0.1% by weight, calculated on the monomer or monomer mixture.