We have investigated the diffusive properties of model metal nanoparticle dispersions in polymeric matrices of several different molecular weights. Rutherford Backscattering Spectrometry was used to measure the depth distribution of gold nanoparticles within thin layers of poly(t-butyl acrylate) (PTBA). The gold nanoparticles were created by evaporation of a discontinuous gold layer onto a thin film of PTBA. A second PTBA film was placed onto these samples to create “sandwiches” in which the gold existed between two PTBA films. Gold particle diffusion coefficients were obtained from measured gold particle depth distributions in annealed samples for which the molecular weights of the two PTBA layers were identical. The experiments revealed that particle mobility was decreased by two to three orders of magnitude compared with the predictions of the Stokes-Einstein model of particle diffusion. These results are attributed to bridging interactions between particles arising from slow exchange kinetics of polymer segments at the polymer/metal interface. Experiments for which the molecular weights of the two polymer films are different are sensitive to the ability of polymer molecules to pass through the gold particle layer. Experiments done with thermally evaporated particles are consistent with a picture in which polymer molecules are able to freely pass through the gold particle layer. Results obtained with gold deposited by electron-beam evaporation are strikingly different. The gold in this case is not able to diffuse, and polymer molecules are not able to penetrate the gold layer. These results, in addition to preliminary results from optical absorption experiments, indicate that much smaller particles are obtained by electron-beam evaporation than by thermal evaporation.