An unsteady state model for ultrafiltration that describes transport within the pores of the ultrafiltration membrane is presented. The new, improved model combines the osmotic pressure model for transport in the polarization layer and the irreversible thermodynamics approach. The model is capable of simultaneously predicting 1) the permeate flux, 2) the permeate concentration, and 3) the concentration at the membrane feed surface when all the transport parameters are fixed. The model data are comparable to the experimental data cited in the literature. Model simulations conducted by varying 1 parameter while the others are kept constant show that the entire range of observed transmissions is a function of permeate flux. For certain parameter values, the model predicts observed transmission greater than unity. Hence, in principle, an ultrafiltration system in which the product of interest is concentrated in the permeate stream can be designed. The model also correctly predicts the experimental observation that the magnitude of observed transmission that is greater than unity decreases as the feed concentration increases.