Ultrafiltration of polyethylene glycol aqueous solutions was analyzed using concentration polarization-gel layer model. The model is based on coupling concentration polarization layer and gel layer growth. The concentration polarization layer development is given by the convective-diffusion equation. However, the gel layer growth model is obtained by using the experimental observations. The limiting flux was found to be proportional to the square root of the axial velocity. Beyond the critical concentration, both concentration polarization and gel formation are the main factors controlling the limiting flux in the tubular UF system. The developed semi-empirical model, which contains a single empirical constant to be evaluated from experiments, predicted satisfactorily the limiting flux data over the range of experimental variables examined in this study. (C) 2009 Elsevier B.V. All rights reserved.