Several polyethersulfone ultrafiltration membranes have been made with small quantities of polyvinyl-pyrrolidone of different molecular weights. Our aim was to increase the permeability without significant changes in selectivity. Different approaches to get effective pore size distributions have been compared between them and with the topographic results obtained from atomic force microscopy (AFM) image analysis. In particular, the use of cumulative retention coefficients: have proved to be preferable to adequately obtain effective pore size distributions: based on retention of multidisperse solutions. Within this frame, it has been proved that small PVP content of the studied membranes leads to different increasing permeabilities with very similar selectivities.