A pseudophase-change solution-diffusion (PPCSD) model, which assumes the pervaporation process is a combination of liquid permeation mechanism and vapor permeation mechanism in series, is proposed. A pseudophase change of permeant is located at the interface between these two mechanisms. Using the assumptions of thermodynamic equilibrium, simple concentration-dependent solubility, and diffusivity, we derive a set of analytical equations to express the pervaporation flux, the lengths of the liquid and vapor permeation mechanisms. The PPCSD model was tested for its validity and showed good agreement with literature data in terms of the effect of feed pressure and permeate pressure. The length of the permeation zone that behaves as a liquid permeation mechanism as well as the pressure and concentration profiles within the membrane also can be calculated. Comparison among three different membrane separation processes (namely, pervaporation, vapor permeation, and liquid permeation) showed that the permeation flux is nearly the same for pervaporation and vapor permeation processes under the assumption of thermodynamic equilibrium. On the other hand, the permeation flux for the liquid permeation process is always lower unless a certain high feed pressure is reached.