The stopped-flow technique was successfully used for directly monitoring the formation of perylene nanoparticles by the reprecipitation method for the first time. Nanoparticles were rapidly formed within tens of milliseconds after mixing of the solvents. By studying the dependence of the rate of particle formation on the supersaturation ratio of perylene, it was demonstrated that the kinetics of the reprecipitation process was in accordance with the Classical Nucleation Theory. As predicted by Such theory, the supersaturation ratio of perylene showed a critical effect on the size of the nanoparticles formed, where higher supersaturation ratios yield smaller particles. (C) 2008 Elsevier B.V. All rights reserved