Starting from the classical Kedem-Katchalsky model for filtration through membranes, we derive an equation to compute the permeate flux and the rejection of diprotic amino acid compounds. We discuss the influence of the pH, the viscosity and the feed concentration of the solution on the transport parameters which characterize the membrane, and we improve the basic equation by accounting for these dependencies. We propose two possible strategies to estimate the transport parameters from experimental data. Finally, we compare our simulation results with experiments of tight ultrafiltration (UF) and nanofiltration (NF). (C) 2009 Elsevier B.V. All rights reserved.