Polymer interdiffusion in a poly(vinyl acetate-co-dibutyl maleate) [poly(VAc-co-DBM) latex film containing ca. 50 wt % gel was studied by Forster resonance energy transfer (FRET). The latex particles were labeled either with an energy donor dye or with an acceptor dye. Films prepared from a mixture of donor- and acceptor-labeled particles exhibit nonexponential donor fluorescence decay profiles, which were analyzed in two distinct ways. In a first simplified approach, we calculated the fractional growth in the quantum efficiency of energy transfer f(m), from which we obtained an apparent diffusion coefficient, D-app, which decreases with increasing annealing time. In addition, we developed a new model for spherical diffusion that accounts for the presence of immobile gel nanodomains within the latex particles. We calculate the energy transfer from the donors in one particle to the acceptors in another, taking into account the concentration profile of the polymer arising from diffusion of the un-cross-linked chains through the gel nanodomains and across the particle boundary. By comparing simulated donor survival probabilities with experimental decay profiles, we obtained the polymer concentration profiles at the interface between particles and the corresponding mean cumulative diffusion coefficients D), describing diffusion of the mobile polymer fraction, as a function of annealing time for different annealing temperatures. From the polymer concentration profile, we calculated the roughness of the initial interface (before any annealing) for gel containing latex particles. This value is much lower than that found for un-cross-linked samples. A comparison of the two methods of analysis shows that Dapp values are up to three times larger than &LANGBRAC; D&RANGBRAC; but track the evolution of the "true" diffusion coefficient fairly well. From the temperature dependence of &LANGBRAC; D&RANGBRAC; we found an effective activation energy for diffusion of 39 kcal/mol.