The dependence of adhesion on the surface segmental dynamics of polymer substrates in thin-film poly(methyl acrylate) (PMA)- and poly(methyl methacrylate) (PMMA)-adhesive tape systems was probed via tape peel tests. From 90 angle tape peel experiments at 28 C of very thin films of PMA adsorbed on glass slides, it was found that the peel velocities varied linearly with applied force and the force-velocity curves were dependent on film thickness. Extrapolation of the peel-velocity curves to zero velocity yielded an estimate of the fracture energies, which varied from about 50 to 20 J/m2 for thicknesses from 50 to 2000 nm. The fracture energies were also found to depend exponentially on film thickness with a decay length of 110 nm. In contrast to the behavior of PMA, PMMA films showed larger fracture energies but no thickness dependence. The differences in the behavior of the two types of films are believed to be due to differences in segmental mobilities in the films which give rise to differences in the motions of the "anchoring sites" across the interface. For PMMA, the glassy polymer was rigid at the interface, regardless of the thickness, and hence, no thickness dependence was observed. For PMA, a rubbery polymer at room temperature, the segments at the adhesive interface were influenced by the restrictions posed by interactions of the polymer with the glass surface. A model based on differing thicknesses of the polymer interphases is proposed to account for the behavior observed.