A 3D hydrodynamic focusing microreactor was used to observe the in situ nanocrystallization of rubrene by fluorescence lifetime imaging (FLIM). The crystallization is achieved by a precipitation method based on the addition of a non-solvent. The rubrene solution flows from a capillary and is squeezed in 3D by two side flows containing water. The principle of the 3D hydrodynamic focusing is detailed; this device allows to follow the in situ crystallization process in less that 5 ms. Moreover, different experiments have demonstrated the possibility of quantifying the dynamics of the crystallization process which is linked with the diffusion process. (C) 2007 Elsevier B.V. All rights reserved.