The surface glass-transition temperature (T-g(s)) in monodisperse polystyrene (PS) films was examined on the basis of the temperature dependence of the lateral force at a given scanning rate. T-g(s) of the PS films was found to be markedly lower than the corresponding bulk glass-transition temperature over the entire molecular weight range (4900-1,460,000). Also, the apparent activation energy of the surface segmental motion in the PS films was 230 +/- 10 M mol(-1), significantly smaller than that for bulk PS. Finally, T-g(s) measurement by lateral force microscopy was applied to binary PS mixtures with different molecular weights. On the basis of the Gordon-Taylor equation, the surface composition of the shorter component in the blend films was successfully obtained. (C) 2003 Wiley Periodicals, Inc.