The ground- and excited-state acidbase properties of three novel naphthalimide-based super photoacids were studied using steady-state and time-resolved spectroscopy. The compounds exhibit pK(a) = 8.88.0 and pK(a)* = -1.2 to -1.9. The decrease in both ground- and excited-state pK(a) is achieved by attachment of an electron withdrawing group (sulfonate) on the aromatic system. All compounds are deprotonated upon excitation in alcohols and DMSO. Good correlation is established between the pK(a)* and the ratio of the neutral and anion emission intensities in a certain solvent. The excited-state intermolecular proton transfer to solvent (H2O and DMSO) is explained by a two-step model. In the first step, short-range proton transfer takes place, resulting in the formation of a contact ion pair. Free ion pairs are formed in the diffusion controlled second step.