A new method has been successfully developed for the control of sizes of organic nanoparticles. The method, which relies on the use of stopped-flow UV-visible absorption spectroscopy to measure the rate of perylene nanoparticle formation by reprecipitation in water, is based on the systematic addition of an organic additive (N,N'-bis(2,6-dimethylphenyl)-3,4,9,10-perylenedicarboxyimide) to the target compound (perylene) solutions. Upon reprecipitation, a strong dependence between additive concentration and rate of perylene nanocrystallization is observed. Authors clearly demonstrated that, in the reprecipitation method, faster rates of organic nanoparticle formation is achieved with increasing additive concentration, and consequently to smaller organic nanoparticles. It is believed that N,N'-bis (2,6-dimethylphenyl)-3,4,9,10-perylenedicarboxyimide acts as "seeds" for further growth of perylene nanoparticles up to approximately 400 nm. (C) 2009 Elsevier B.V. All rights reserved.