Agglomeration in suspension is a size-enlargement method which allows the formation of spherical agglomerates of a few millimeters from fine particles (< 100 mu m) by the action of a binding liquid. This paper proposes a new technique to measure the particle size distribution in order to determine agglomeration kinetics. The device, which consists of a video camera focused on a black hat screen fixed inside the crystallizer and connected to an image analysis system, films and measures in situ the particles circulating between the screen and the transparent crystallizer wall. This visual method also gives an interesting insight into agglomeration mechanisms.