Surface molecular aggregation structure and surface molecular motions of high-molecular-weight polystyrene/low-molecular-weight poly(methyl methacrylate) (HMW-PS/LMW-PMMA) blend films were investigated on the basis of X-ray photoelectron spectroscopic measurements and scanning force microscopic observations. Monodisperse PS with M-n = 1450k, where M-n denotes the number-average molecular-weight, and monodisperse PMMAs with M-n 1.2k, 4.2k, 40.5k, 144k, and 387k were used as HMW-PS and LMW-PMMAs, respectively. Static contact angle measurements revealed that the magnitudes of surface free energy, gamma, of PMMAs for all M(n)s studied here were higher than that of PS with M-n = 1450K. In the case of the (HMW-PS/LMW-PMMA) blend films, in which the M-n for each PMMA was less than 144K, PMMA was preferentially segregated at the air-polymer interface, even though PMMA had a main chain with a higher gamma compared with that of PS. It was found from scanning viscoelasticity microscopic measurements that the surface molecular motion of the (PS with M-n = 1450k/PMMA with M-n = 4.2k) blend film was fairly activated in comparison with that of the bulk one due to the surface segregation of LMW-PMMA. The surface enrichment of LMW-PMMA can be explained by enthalpic and entropic terms as follows. (1) Since the magnitudes of gamma of both chair! end groups of a polymer chain synthesized by an ordinary living anionic polymerization are smaller than that of the main chain part, the chain end groups are preferentially segregated at the surface. Therefore, the chain end effect at the air-polymer interface becomes more remarkable with a decrease of M,, due to an increases in the number density of chain end groups. (2) Since polymeric chains existing in a surface region are compressed along the direction perpendicular to the film surface, the surface chains take smaller conformational entropy in a confined state compared with that of bulk chains. The difference in conformational entropy between the surface chain and the bulk one, that is, the conformational entropic penalty of the polymeric chain at the surface, decreases nith a decrease in M-n. Then, when the enthalpic and entropic effects mentioned above overcome the gamma difference of main chain parts between PS and PMMA, PMMA with higher gamma is stably enriched at the blend film surface.