A solvent-exchange process was applied to three fluorescent dyes belonging to the 2-benzimidazolyl-7-diethylaminocoumarin series (namely Coumarin 7 (1), Coumarin 30 (2), and one of their derivatives bearing a butyl chain (3)). The three compounds only differ by the substitution of the nitrogen atom of the benzimidazolyl group. They were first dissolved in acetone and then suddenly placed in an aqueous environment where they generated molecular assemblies. The size and shape of the latter were studied by fluorescence microscopy and transmission and scanning electron microscopy. It appeared that 1 gave aggregates and flat microcrystals that evolved toward elongated structures. 2 formed straight nanofibers that are 10-20 mu m long and exhibit a crystal structure. 3 gave short fibers (1 mu m x 25 nm), which finally arrange into entangled solid nanofibers. The formation of fibers arising from low-molecular-weight molecules was particularly interesting. The optical properties of the free-standing particles in suspension were analyzed and compared to those of highly dilute dyes, with the aim to get additional information about the dye arrangement in the nanostructures.