We report measurements of the kinetics of polymer diffusion in latex films prepared from two pairs of poly(butyl methacrylate) (PBMA) latex particles. One set contains only a small amount of sulfate groups at the surface; the other contains a significant amount of carboxylic acid groups at the surface. Neutralization of these acid groups with ammonium, sodium, and barium hydroxide retards the interdiffusion rate at 100-degrees-C 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 latex 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 both metal ions are much more effective than NH4+. In the case of the carboxylated latex, this may point to loss of NH3 during film annealing, regenerating free COOH groups.