Sulphur-free phenols and quinones are formed during fatiguing of peroxide-vulcanised natural rubber containing phenolic sulphides (I). Only phenols are formed during vulcanisation and it is shown that the derived quinones are the main secondary antifatigue agents. However, none of these, including galvinoxyl, is as effective as the parent sulphides. The evidence suggests that quinonoid products trap macroalkyl radicals, becoming chemically attached to the polymer in the process. The high activity of the phenolic sulphides is primarily due to sulphur-containing oxidation products.