A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound I is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at lambda(max) = 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of I by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of I is highly sensitive to changes in surfactant concentration. This renders I a versatile and sensitive probe molecule for studying the inicellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.