The invention provides a composition comprising a mixture or reaction mixture of a carbodithioic acid of formula: (I) or its internal salt; and at least one compound independently selected from a natural or synthetic rubber curing or vulcanization accelerator, activator or retarding agent. wherein R′ and R″ are independently selected from a C2-C18 aliphatic moiety; aromatic moiety; alicyclic moiety; aromatic heterocyclic moiety; and aliphatic heterocyclic moiety or R′ and R″ may together form part of an aromatic heterocyclic ring moiety or aliphatic heterocyclic ring moiety.