The effect of molar mass and gel content on the rate of polymer diffusion in film based on poly(vinyl acetate-co-butyl acrylate) P(VAc-BA) latex particles with a 4:1 weight ratio of VAc and BA is examined by fluorescence decay measurements of nonradiative energy transfer (ET). P(VAc-BA) latex particles labeled with phenanthrene (Phe) as the donor and with 4'-(dimethylamino)benzophenone (NBen) as the acceptor were prepared by semicontinuous emulsion polymerization in the presence of different amounts of chain transfer agent to modify the mean molar mass and gel content of the latex particles. We synthesized three sets of samples, which we refer to as high M, medium M, and low M. The high M sample had about 40% gel content, and the sol fraction extracted from the polymer had a nominal M-w (GPC) of 125 000. The medium M and low M samples had a negligible gel content and corresponding values of M-w approximate to 75 000 and 42 000, with a broad size distribution. The rate of polymer diffusion in these films was found to increase strongly with decreasing mean molar mass. The diffusion rate increased with annealing temperature. For the high M sample, we found that the diffusion rate was characterized by an apparent activation energy E-a = 37 kcal/mol. This value is somewhat larger than that (E. = 34 kcal/mol) reported previously for the medium M sample determined both by ET experiments and by viscoelastic relaxation measurements. The extent of mixing parameter f(m) showed a power law dependence on diffusion time, with f(m) similar to t(z), with z values on the order of 0.12-0.18.