During ultrafiltration, a transmembrane pressure difference is applied across a polymeric membrane. This pressure difference results in solute transport through the membrane and mechanical compression of the membrane. The ultrasonic time-domain reflectometry (UTDR) technique is used to measure mechanical compression of the membrane structure in real time while water permeability is simultaneously measured during ultrafiltration in a specially designed crossflow filtration cell. The membranes used in this study are Zirfon(R) organo-mineral membranes fabricated using 0.9-mum zirconia particles at three different loadings. Thickness changes and the permeability of composite ultrafiltration membranes are obtained simultaneously and in real-time as a function of filler concentration and transmembrane pressure. Results indicate that the presence of ZrO2 particles in the polymeric matrix influences the membrane permeability as well as compression of the membrane structure. Increasing the filler concentration decreases the elastic strain component but increases the lime-dependent strain component. Both the initial and the time-dependent strain increase with increasing pressure. The Aux decline is more severe at higher pressures and appears to be due to localized deformation in the membrane skin layer. This study provides the first quantitative information regarding compaction of organo-mineral Zirfon(R) membranes. (C) 2001 Elsevier Science B.V. All rights reserved.