Neutron reflectometry is used to study interdiffusion in bilayers of hydrogenated and deuterated poly(methyl methacrylate) (PMMA) an silicon substrates where the polymer-substrate interaction energy is strongly attractive. The effect of the polymer-substrate interaction energy on the interdiffusion rate as a function of distance from the surface is investigated. Samples are prepared with lower layer thicknesses of the deuterated PMMA ranging from 35 to 335 Angstrom or 0.4 to 3.6 radii of gyration (R-g) of the bulk polymer chain. The rate of interdiffusion is found to he strongly dependent upon the lower film size. Polymers in films less than R-g in thickness have effective diffusion constants 2 orders of magnitude smaller than polymers in the thickest films. The effective range of the substrate on the interdiffusion dynamics is found to be between 300 and 400 Angstrom or 3 R-g and 4 R-g. The observed dynamics are attributed to the different polymer chain conformations at varying distances from the solid surface.