The thermal stability of poly(methyl methacrylate) (PMMA) composites with the two fullerenes C-60 and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), was studied over the whole composition range under nitrogen (N-2) and air atmosphere using dynamic thermogravimetric analysis (TGA). The aim of this study was to compare the thermal stabilization effect of both fullerenes on PMMA. When compared with pure PMMA, both C-60 and PCBM increase substantially the thermal degradation temperature of the corresponding composites, both in nitrogen and air atmospheres. The amounts of C-60 and PCBM in the PMMA composites can be successfully determined along the whole composition range using TGA. The initial amounts of fullerene present in the composites correlates linearly with the residual mass values obtained above 450 degrees C in nitrogen and with the residual mass values observed at the temperatures of transition from the first to the second decomposition steps in air. There are no significant differences between the thermal stabilization effects of C-60 and PCBM in both atmospheres (nitrogen and air).