The mechanisms of fouling in the ultrafiltration of polyethylene glycol (PEG) are analysed using the complete blocking and the intermediate blocking Hermia's models adapted to crossflow filtration. The parameters of these models were theoretically estimated. The predicted results were compared with experimental data. Ultrafiltration experiments were performed with Carbosep M2 monotubular ceramic (Orelis, S.A. (France)). The fouling ultrafiltration experiments were carried out at a constant temperature and feed concentration and different feed flow rates and transmembrane pressures. The precision in the predictions is very high. The results showed that the phenomenon controlling fouling was intermediate blocking for high fouling conditions. (C) 2011 Elsevier B.V. All rights reserved.