Considerable theoretical and experimental work has been published to explain the cause of flux decline in the ultratiltration (UF) of dextran. However, an experimental determination of resistance contributions due to polarization to the overall filtration resistance has not been addressed. Also, the effects of feed concentration and applied pressure on membrane concentration and boundary layer thickness and unsteady-state flux response to a step change in trans-membrane pressure have not been reported. This work reports experimental results of the ultrafiltration of dextran with a fully retentive membrane along with a procedure to determine the contributions of various resistances to filtration. It was observed that the ultrafiltration of dextran was osmotically limited in our work. The contribution of polarization resistance was found to be dependent on applied pressures and initial bulk feed concentrations whereas the boundary layer thickness of polarization was affected by feed concentration alone. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.