A novel optical technique is used to investigate the dynamics of coagulation of particle suspensions with aluminum salt. The relative size of aggregates formed during coagulation, expressed as a ''coagulation index,'' can be continuously measured by this technique. Coagulation of clay particles with aluminum sulfate is examined at various aluminum concentrations, solution pH, and mixing intensities. Results indicate that the coagulation index provides valuable information about aggregate dynamics and coagulation mechanisms with aluminum salts.