An in-situ image acquisition and analysis system was developed to characterize the dynamics of particle size distribution (PSD) during flocculation. The system consisted of a flocculator coupled with an external flow-through cell for observation and photography, a microscopic charge-coupled device video recorder with backlighting, and an image analyzer. This non-intrusive side-stream setup was used to record the evolution of the PSD to determine the flocculation dynamics of two types of particle systems: Clean kaolin, kaolin coated with natural organic matter (NOM). The results were discussed and the error induced during measurement was evaluated.