This paper investigated the thermal behavior of polyarylsulfone (PSF) films aged by hydrochloric (HCl) and nitric acids (HNO₃). The thermal decomposition and glass transition of PSF films were evaluated using thermogravimetry (TG) and differential scanning calorimetry (DSC). The TG curve of untreated PSF film shows a two-stage process, while TG curves of aged films show a three-stage process. The weight loss which was found in the aged films at low temperature region, below 300°C, is considered to be due to the decomposition of PSF with low molecular weight. This suggests that a part of PSF is decomposed by HCl and HNO₃. The glass transition temperature (T_g) of PSF decreased as the period of treatment in the acids was increased. However, T_gs of the samples increased when the samples were immersed in 15% HCl, concentrated HCl and 15% HNO₃ for more than 120 days. The decrease of T_g showed a good linear relationship with the increase of weight loss near 230°C.