In this work Hermia's models adapted to crossflow ultrafiltration were used to investigate the fouling mechanisms involved in the ultrafiltration of polyethylene glycol (PEG). Although PEG has been very often used as a standard macromolecule in ultrafiltration experiments to test flux decline models, it has not been used to test Hermia's models adapted to crossflow ultrafiltration. This work analyses the different fouling mechanisms that may occur at different stages of the ultrafiltration process. Fitted Hermia's models parameters are also analyzed for the experimental conditions tested on the basis of their physical meaning. Ultrafiltration experiments were performed with ceramic membranes supplied at different experimental conditions: feed flow rates and transmembrane pressures (TMPs). The results showed that the phenomenon controlling fouling was intermediate blocking for the highest TMP and the lowest crossflow velocity tested. For lower TMPs and/or higher a crossflow velocities, complete blocking or intermediate blocking controlled the fouling process. The analysis of the fouling mechanisms performed dividing the filtration curves in different regions revealed that complete blocking was dominant for a TMP of 0.3 MPa and a crossflow velocity of 1 m/s and in the case of a TMP of 0.4 MPa and a crossflow velocity of 2 m/s. (C) 2008 Elsevier B.V. All rights reserved.