The reprecipitation method is a simple and useful way to prepare microcrystals through a solvent exchange process. It was applied to three fluorescent dyes of the 4-amino-7-nitrobenz-2-oxa-1,3-diazole series. Compounds 1, 2, and 3 differ by the length of the alkyl chain, which comprises 8, 12, and 18 carbon atoms, respectively. The reprecipitation process was first studied in water, in the absence of additives. The kinetics was monitored by UV/vis absorption spectroscopy. The size and shape of the microparticles were analyzed by fluorescence microscopy and transmission electron microscopy. Dyes 1 and 2 lead to microcrystals, the whole process taking much more time for 2 than for 1. The long-chained dye 3 only gave stable aggregates. Therefore, it appears that the hydrophobicity of the organic dye markedly influenced the reprecipitation process. The latter was then studied in the presence of additives. Poly( amidoamine) dendrimers, terminated by 64 carboxylate or amino groups were placed in the reprecipitation medium. They had little effect upon the formation of aggregates for dye 3. In contrast, they drastically accelerated the reprecipitation of 1 and 2 and tuned the size and shape of the microcrystals. Platelets and spindles were thus obtained by varying the nature of the dendrimer, and their optical properties were briefly investigated.