Isothermal physical aging and the glass transition temperature (T (g)) of PMMA thin films were investigated by means of differential scanning calorimetry (DSC). Freestanding thin films of different molecular weights (M (w) = 120,000, 350,000, 996,000 g/mol) and film thicknesses (40-667 nm) were obtained by spin coating onto a silicon wafer substrate and then releasing the coated film using a water floating technique. The thin films were stacked in a DSC pan and isothermally aged for different aging times (t (a) = 1 and 12 h) and aging temperatures (T (a) = 105, 110, and 115 A degrees C) below but near T (g). Enthalpy relaxation (Delta H (Relax)), resulting from the isothermal physical aging, initially increased with increasing Delta T (T (g) - T (a), driving force of aging), reached a maximum value, and then decreased with further increase in Delta T. Below similar to 100 nm film thickness, Delta H (Relax) of samples aged near their T (g) (i.e., T (a) = 110 and 115 A degrees C) decreased with decreasing film thickness, indicating the suppression of physical aging. Up to 9.9 A degrees C depression in T (g) was observed for thinner films (similar to 40 nm), when compared to the thicker films (similar to 660 nm) in this study. The decrease in Delta H (Relax) with decreasing film thickness at a given T (a) appears to be associated with the reduction in T (g).