The transport of solvent out of a cast cellulose acetate (CA) solution into the coagulation bath during membrane formation is treated as a diffusion process. From the increase of solvent concentration in the bath with time (solvent leaching experiments) an overall solvent diffusion coefficient has been calculated. In size these coefficients compare well to mutual pseudo-binary solvent-non-solvent diffusion coefficients determined by means of a classical boundary broadening method applied to ternary solutions with fixed CA concentration, but with a gradient in solvent-nonsolvent composition. Since binary polymer-solvent interdiffusion coefficients are at least one order of magnitude lower, it is concluded that the diffusion of solvent into the coagulation bath is essentially a pseudo-binary solvent-non-solvent diffusion process. Combination of experimental results with model calculations for the effect of a thin dense skin on the diffusion of solvent out of the sublayer shows that the casting-leaching diffusion coefficient can be used to describe the out-diffusion of solvent from the layer under the skin provided that the relative skin resistance is not too high, or that the skin thickness is small.