This study evaluates differences in reversible (after relaxation) and irreversible compaction and the effect of compaction on the performance of three different ultrafiltration membranes. The evaluation is based on results from both off-line and on-line measurements of compaction. The on-line measurements were done with an ultrasonic time-domain reflectrometry (UTDR) tool with improved resolution compared to tools used in earlier studies. The results reveal that the regenerated cellulose membrane compacted significantly more than the tested polyethersulphone membranes. This dissimilarity originates from the different membrane materials used and from significant differences in the membrane structures. It is also found that measurements of membrane compaction, whether made on-line or off-line, are not predictive for membrane performance. For instance, compaction of the UH030 membrane was negligible but its permeability decrease and retention increase due to the compaction were significant. Compaction decreased the cut-off values of the 30 kDa membranes to lower than 8 kDa. The results thus indicate that the skin layers of the membranes compact significantly causing remarkable changes in membrane performance. Thickness changes occurring in the scale of skin layer thicknesses are out of the resolution limits of methods thus far available for monitoring of membrane compaction in real-time. Real-time measured information on compaction phenomena is further needed to be able to distinguish flux decrease caused by concentration polarization and the effects of reversible compaction. (C) 2013 Elsevier B.V. All rights reserved.