Neutron reflectometry is used to measure polymer mobility over distances less than 200 Angstrom as a function of distance from an attractive solid surface and molecular size. Effective diffusion coefficients, D-eff, are determined from the rates of interdiffusion between supported bilayers of deuterated and hydrogenated poly(methyl methacrylate) (PMMA). The thickness of the d-PMMA layer is varied from less than the radius of gyration, R-g of the bulk polymer to 11R(g) for three molecular weight pairs of polymers. D-eff dramatically decreases in samples with d-PMMA layers less than 3R(g) thick and approaches bulk interdiffusion rates from layers greater than 5R(g) thick.