A cross-flow ultrafiltration model which couples the formation of a cake layer and the existence of a polarized layer above the cake has been developed. The model contains three constants and the specific cake resistance to be evaluated from experiments. The model gave an excellent prediction on the performance of the cross-flow ultrafiltration of polyethylene glycol solutions. Experimental observations on the effects or transmembrane pressure, solute concentration and cross-flow velocity agreed well with the predicted permeate fluxes.