A novel mixture comprises a dihydrocarbyl xanthogen polysulphide and a xanthate selected from metal hydrocarbylxanthates and dihydrocarbylxanthates. Such a mixture can be used to cure a vulcanisable rubber composition containing less than 0.4 parts by weight of nitrosatable materials per 100 parts by weight rubber; the use of toxic conventional nitrogen-containing accelerators such as amines can be avoided.