The degradation of isotactic and syndiotactic poly(methyl methacrylate) (PMMA) was studied by thermogravimetric analysis (TGA) with an emphasis on their behavior in ultrathin films on silica. Both PMMA tacticity and adsorbed amounts were found to affect the degradation. In bulk, syndiotactic PMMA (syn-PMMA) has higher thermal stability than,isotactic PMMA (iso-PMMA) due to its lower chain mobility. The T-max (maximum rate decomposition temperature) is lower than that of bulk samples at higher adsorbed amounts for both iso- and syn-PMMA. T-max increases when the adsorbed amount on the silica surface decreases for syn-PMMA. The degradation behavior of adsorbed iso-PMMA became very complex at low adsorbed amounts.