The glass-transition temperatures (T-g's) of nanocomposites of polystyrene (PS) and single-walled carbon nanotubes were measured in the bulk and in thin films with differential scanning calorimetry and spectroscopic ellipsometry, respectively. The bulk T-g of the nanocomposites increased by approximately 3degreesC and became much broader than that of PS. For the nanocomposite films thinner than 45 nm, T-g decreased with decreasing film thickness [i.e., DeltaTg(nano)<0]. This phenomenon also occurred in thin PS films, the magnitude of the depression in PS [&UDelta;T-g(PS)] being somewhat larger. The film thickness dependence and the differences in the magnitude of &UDelta;T-g in the two systems were examined in light of current theory, and a quantitative comparison was made. (C) 2003 Wiley Periodicals, Inc.