Self-diffusion coefficients of water in inhomogeneous :materials-porous cellulose acetate membranes (MF: average pore sizes 0.025, 0.8 and 8.0 mu m) and ceramic membrane (CM: average pore size 4.4 mu m)-were measured as a function of diffusion time using PFG-NMR. The relationship between the water diffusion coefficient and the membrane structure, as determined by SEM, was examined. The magnitude of the water diffusion coefficient is in the order of MF8 > MF0.8 > CM > MF0.025. These were well explained by the restricted diffusion theory of Tanner. According to this theory, the diffusion coefficients depend on the pore size and thickness of the membrane matrix. Results show that the decrease in the diffusion coefficient is affected not only by a decrease in the pore diameter but also by an increase in the membrane matrix size.