The possibility of a double log-normal (bimodal) permeability distribution for ultrafiltration membranes is discussed and the existence demonstrated by plotting the number of water flux observations against the logarithm of the pure water fluxes. Ultrafiltration membrane water fluxes were also simulated using the Monte Carlo method assuming double log-normal distributions of the pore size. Some explanations of the phenomenon of double log-normal distributions are proposed, based on the presence of two independent distributions : one of the spacing between the subunits, and one of the pores within the microporous subunits that make up the material, for instance, an ultrafiltration membrane. The importance of the membrane thickness and its variation is also pointed out. A thick membrane has lower permeability than a thin at a given pore size distribution.