The sweeping gas membrane distillation process was analyzed in a plate and frame membrane module, and a method clarifying the contribution of each boundary layer separately was developed. The temperature polarization coefficients of the fluid phases as well as the overall one have been defined, and their effects on the permeate flux were studied. Also studied is the influence of experimentally relevant parameters, such as the inlet temperatures or circulation velocities of the fluids. Results from two porous and hydrophobic membranes in different experimental conditions were interpreted oil the basis of a combined Knudsen/molecular diffusion flow model. The theory agreed well with the experiment.