Effect of gamma-irradiation on the thermal stability of poly(methyl methacrylate) with added amounts of cobalt(III) acetyl acetonate has been examined. ?he thermal degradation behavior, studied by thermal volatilization analysis (TVA) and subambient TVA technique showed the formation of methanol, carbon dioxide and the monomer itself. The main effects of the additive result from complex formation with the unsaturated chain ends and from attack of acetylacetonate radicals on the polymer.