Combinations of para-phenylene diamines and para di(nitroso) arenes act synergistically in guayule rubber-based compositions to protect them against oxidative degradation. A typically useful diamine is N-(1,3-dimethylbutyl)-N'-phenyl-phenyl-p-phenylene diamine and a useful arene is para-di(nitroso)benzene. Vulcanizable compositions, as well as articles made therefrom, are within the scope of the invention.