The separation of organic solutes by ultrafiltration, nanofiltration and reverse osmosis is evaluated by a steric pore flow model integrated with the boundary layer theory. The modeling of mass transfer at the fluid phase adjacent to the membrane accounts for the effect of fluid dynamics, variation of feed physical properties and mass transfer rates. The membranes were characterized in terms of an average pore radius. The solute fluxes and separations were assumed to be governed by steric and hydrodynamic forces. The permeation experiments were performed with cellulose acetate membranes and dilute aqueous solutions of reference solutes of increasing molecular weight [alcohols, sugars, poly(ethylene glycols) and dextrans]. The flat sheet membranes were prepared in the laboratory with hydraulic permeabilities ranging from 0.65 to 72.2 kg/h m2 bar.