In some published experimental results on ultrafiltration of proteins, steady-state values for observed protein transmission greater than 100% were reported, i.e, the concentration of the protein in the permeate was found to be greater than that in the feed. Different models, like: combined concentration polarization-irreversible thermodynamics model and modified concentration polarization models were used in order to explain this phenomenon. In this work, we present an unsteady-state model that is capable of explaining this phenomenon. Further, the model is capable of predicting the steady-state permeate concentration if the transport parameters through the membrane are known a priori. The present model is shown to be capable of predicting the entire range of observed transmission as a function of permeate flux.