The thermal and thermooxidative degradation of free-radical atactic and anionic syndiotactic and isotactic PMMAs and molecular blends of these polymers with the C 60 fullerene was studied by mass spectrometry, thermogravimetry, and DSC. It was shown that C 60 inhibits both thermal and thermooxidative degradation of these polymers independently of their structure. It is suggested that fullerene-containing polymers are produced upon the thermal and thermooxidative degradation of PMMA. This fact together with the nonchain inhibition of the thermooxidative degradation of PMMA accounts for the stabilizing effect of fullerene. The addition of the products of thermal degradation and thermal oxidation to C 60 significantly widens the temperature interval in which fullerene itself undergoes thermal oxidation.