Abstract—: Potentials of the electrooptical method for studying fast coagulation have been considered. Results of studying particle aggregation in aqueous diamond and graphite colloids containing AlCl₃ as an electrolyte have been represented. A comparative analysis has been performed for the aggregation rates of diamond and graphite particles at low values of their electrokinetic potentials. It has been found that, at a low electrokinetic potential, a change in its value by a few millivolts alters the stability of diamond and graphite colloids by several times. The investigations have shown that, at the isoelectric point, the formation rate of paired aggregates is five times higher for graphite particles than that for diamond particles at close particle number concentrations. This indicates significant differences in the surface structural components hindering aggregation of graphite and diamond particles. The electrooptical method has been employed to measure the relaxation dependences of electrooptical effects corresponding to different time periods elapsed after the onset of colloid coagulation. These dependences have been used to determine the size distributions functions for particles and their aggregates and to analyze variations in these functions in the course of coagulation. It has been shown that the used method is efficient and applicable for studying the kinetics of fast coagulation.