Many applications of polymer thin films are determined by the molecular structure and chain mobility at the polymer-air surface. In ultrathin polymer films where the film thickness is comparable to the size of the macromolecule, chain connectivity, for example, may mediate effects of the substrate to the polymer-air surface, giving rise to suppression of the surface segmental mobility. In a previous study, we showed that atomic force microscopic adhesion measurements (AFMAM) could be used to quantify the dynamics in the upper similar to 10 nm surface layer of polymer thin films near the glass-to-rubber transition. In this study, we use AFMAM to investigate the surface dynamics of ultrathin films of poly(tert-butyl acrylate) (PtBuA), to look for any systematic variation in the surface chain mobility as the film thickness, d is varied from similar to1 to 13 . Suppression in the tip-sample adhesion was observed when d became < 20 nm (similar to 2 ), suggesting that the extent of the substrate effect is of the order of the size of a macromolecule in the present system.