To operate a membrane plant it is not only important to know everything about reachable fluxes, recoveries and retentions. Especially for the treatment of surface water it is also significant to know how to monitor continuously the integrity of the capillary membranes. For this reason the IWW Rheinisch-Westfaelisches Institut fuer Wasserforschung, Institute at the University of Duisburg, and the Wasserwerk des Kreises Aachen (WdKA) are realising a project supported by the German Federal Ministry for Education, Science, Research and Technology (BMBF), to investigate the use of particle counters as on-line monitoring tool. To allow the detection of defect capillaries also in case of relatively clean water, a mathematical relationship between the quality of the raw water, the number of defect capillaries in an X-Flow (X-flow BV, Bedrijvenpark Twente 289, 7602 KK Almelo, NL) membrane system and the retention of particles to be expected was determined. By this relationship the operator is able to predict how sensitive the counter has to be and how many membrane modules can be monitored by only one counter.