A significant limitation in the application of membrane filtration is fouling caused by particle deposition. To better understand the key mechanisms of particle deposition, a microfluidic filtration system was developed that combined confocal scanning laser microscopy (CSLM) and a dual syringe pump flow control setup. It has been used successfully to visualize in real-time and quantify the deposition of latex particles (0.4 mu m) onto membranes during cross-flow ultrafiltration at two different KC1 concentrations (0.01 M and 1 M). Particle deposition was initially present on the membrane as individual particles, which when deposited, acted as seeds for further particle deposition for both KCl concentrations. Aggregates were only observed after individual particles had already been deposited. After 45 min filtration, only monolayer coverage was observed, despite the formation and deposition of aggregates on the membrane. The main deposition on membranes was mostly of individual particles at 0.01 M KC1, and of large aggregates at 1 M KC1. This difference in particle deposition behaviour can be attributed to the electrostatic changes in the particle-particle and particle-membrane interactions. These findings have important implications for fouling in real systems, where the control of electrostatic interactions may be used to optimize filtration processes.