The photophysics of two derivatives of 4-aminonaphthalimide have been studied in aqueous, ethanolic, and mixed aqueous/ethanolic solvents, including both normal and deuterated solvents. It is found that the fluorescence quantum yield and lifetime both decrease with increased water content of the solvent and that this is entirely due to increased nonradiative decay, the radiative rate constant being virtually independent of the solvent composition. It is proposed that a mechanism involving the formation of a hydrogen-bonded water cluster is responsible for the observed behavior with the excitation energy of the naphthalimide being distributed amongst the stretching vibrations of the water cluster. The increase in the rate of nonradiative decay is greatly reduced in deuterated solvent mixtures in accord with Siebrand’s theory of radiationless processes.