Neutron reflectometry is used to measure polymer mobility over distances less than 200 A as a function of distance from an attractive solid surface and molecular size. Effective diffusion coefficients, Deff, 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, Rg, of the bulk polymer to 11Rg for three molecular weight pairs of polymers. Deff dramatically decreases in samples with d-PMMA layers less than 3Rg thick and approaches bulk interdiffusion rates from layers greater than 5Rg thick.