The role of water in polymer diffusion in latex Rims was examined. For a hydrophobic polymer (poly(butyl methacrylate), PBMA), water has little influence on the polymer diffusion rate. Water absorption in PBMA latex films does increase film turbidity, both for nascent and for well-annealed films. For a hydrophilic polymer (a copolymer of 5 wt % methacrylic acid and 95 wt % butyl methacrylate, P(MAA-co-BMA)], the presence of water (e.g., 5 at %) increases the diffusion coefficient by a factor of 5 at 60 degrees C. Upon neutralization of the carboxylic acid groups of P(MAA-co-BMA) with NaOH, the polymer diffusion is much retarded in dry films but greatly enhanced in wet films : the polymer diffusion coefficients for the wet films are about 2 orders of magnitude larger than those for the corresponding dry films. Neutralization with NH3 results in intermediate diffusion rates, between those of unneutralized and NaOH-neutralized films under both dry and wet conditions.