Bales of antioxidant-stabilized guayule rubber underwent significant losses in bulk viscosity when stored under ambinent comditions at an Arizona processing facility. Previous work implicated the unsaturated fatty acid esters present in entrained resin as promoters of such degradation. Ageing studies as functions of storage temperature and unsaturated acid ester content were carried out on both bulk rubber and rubber-resin solution (miscella) for cultivar AZ-101. The rate of thermo-oxidative degradation, determined as a function of Mooney viscosity, was increased by the addition of unsaturated ester. Anaerobic degradation was not affected in this way. The activation energy of chain thermolysis was found to be 116 +_ 6kJ/mol (28 +_ 1kcal/mol). At temperatures above 60 C, chain recombination successfully competed with chain scission in lower viscosity rubber. The rate of thermo-oxidative degradation was substantially higher in a typical process miscella. A hindered amine antioxidant significantly reduces chain scission under these conditions.