Abstract: Viscosity effects on water entrainment into W/O emulsion drops and permeation-swelling in a (W/O)/W system were studied theoretically and experimentally. The entrainment swelling was found to decrease with increasing emulsion viscosity when this viscosity exceeded 50-100 mPa·a, whereas there was no effect below that range. The dependence on the emulsion viscosity is 0.3 power in moderate-viscosity region(100-600mPa·s)and 0.6 power in high-viscosity region. The permeation-swelling rate decreased as the membrane-phase viscosity increased because of decreasing diffusion coefficient of water in the membrane phase. The mathematical models for estimating the swelling ratio in various viscosity regions was proposed and the predicted values agreed well with the experimental.