Permeability coefficients P of KCl through porous cellulose triacetate (CTA) membranes were measured as a function of the water volume fraction V-W and diffusion coefficients D were determined using solubility parameters K and a membrane thickness d from the relationship of P = KD/d. D increased with an increase in V-W. D especially increases abruptly around V-W = 0.5, which corresponds to 2% triethylene glycol (TEG) content. The percolation theory was applied to the experimental results under the conditions D-A = D (V-W = 1) = 1.8 x 10(-5) cm(2) s(-1), D-B = D(V-W = 0) = 1.8 x 10(-8) cm(2) s(-1), coordination number (Z) = 2.5, 3, 3.5, and 4, and packing fraction f = 1.0. A good fit was obtained at Z = 3.5 because the experimental and calculated results also shifted at the same V-W below V-W = 0.5. It is suggested that a phase inversion, that is, change of a discontinuous water phase to a continuous water phase, occurs around V-W = 0.5. Above V-W = 0.5, the experimental results agree well with the calculated line for Z = 3 or Z = 2.5 which means that the coordination numbers decrease with an increase in water content. It is thought that V-W is overestimated because it is hard to completely wipe off the excess water quickly from the membrane surface. Z = 3.5 means that a pore can connect in 3.5 directions.