The detailed structures and distributions of the deposit layer formed during cross-flow filtration of 0.4 mu m polystyrene particles has been investigated. A new filtration system was used to directly visualize particle deposition on the membrane surface in a filtration channel in real time. High-resolution and time sequenced images were obtained as well as 3D views of the filtration channel. The structural differences of the deposit layer over a range of solution conditions (pH, ionic strength, and feed concentration) were determined to yield new insight into the fouling mechanism. The time-dependent deposition behaviour was characterized on the basis of surface coverage, deposition volume and normalised deposition volume. The initial deposition process was primarily governed by membrane-particle interactions, while the characteristics of the fully developed deposition layers were mainly affected by particle-particle interactions.