The present invention relates to a process and composition for the chain stopping of vinyl chloride polymerization or copolymerization in the presence of a stable free nitroxyl radical and a phenolic antioxidant. A further subject of the invention is the use of a stable free nitroxyl radical together with a phenolic antioxidant as a chain stopping mixture for the vinyl chloride polymerization or copolymerization. A further aspect of the invention is the use of higher sterically hindered nitroxyl radicals alone as chain stopping agents for the vinyl chloride polymerization.