We report measurements of the kinetics of polymer diffusion in latex films prepared from pairs of poly(butyl methacrylate)[PBMA] latex particles. One set contains only a small amount of sulfate groups; the others contain significant amounts of carboxylic acid groups at the surface. Neutralization of these acid groups with sodium and barium hydroxide retards the interdiffusion rate but does not suppress it. In these experiments, we employ pairs of particles identical in size, with very similar molecular weights, molecular weight distributions, and surface charge densities, that differ only in the fluorescent chromophore (phenanthrene [Phe] or anthracene [An]) used to label the polymer. Each pair of latexes allows us to use direct nonradiative transfer [DET] experiments to follow polymer interdiffusion in films prepared from their dispersions. Acid group neutralization in the latex converts an acid-rich phase at the interparticle boundary in the film into an ionomer phase. Ba2+ salts are more effective than Na+ at slowing down the polymer diffusion rate, and their influence increases in proportion to the amount of polar groups present. We follow an approach developed by Eisenberg (Macromolecules 1971, 4, 125) to estimate the T-g values of the ionomer phase in each film and show that there is a linear relationship between the mean apparent diffusion coefficient of the polymer determined from the DET experiment and T - T-g, where T is the annealing temperature of the film.