A process is provided for rapidly and non-destructively evaluating the solute retention characteristics of porous membranes using a novel liquid-porosimetric technique. A ratio of two membrane permeabilities is measured at preselected operating conditions using a pair of mutually immiscible fluids, one of which is employed as a membrane wetting agent and the other used as an intrusion fluid. At the first operating condition, a particular transmembrane pressure is chosen so as to intrude nearly all of the pores present in the membrane sample. At the second operating condition, a particular membrane permeability is chosen so as to achieve a specified permeability ratio. The resulting transmembrane pressure corresponding to this particular membrane permeability is compared to a previously established standard curve which permits identifying the solute retention characteristics of the porous membrane.