The protonated cation of 2-(2'-hydroxyphenyl)benzimidazole becomes a very strong acid in its first excited singlet state (C*). We studied the proton-transfer process from C* to the bases water, methylurea (MU), and dimethyl sulfoxide (DMSO) in acetonitrile solution by means of fluorescence and UV-vis absorption spectroscopy. We found that the process occurs via a 1:1 hydrogen-bonded adduct between the photoacid C* and the base. We determined the photophysical properties of the adducts with the three bases and the equilibrium constants of formation of various adducts in the ground and excited states. The proton transfer takes place by dissociation of the adduct, in a unimolecular or bimolecular process involving a second molecule of the base. The unimolecular dissociation takes place for the adducts formed with DMSO and MU, but not for the adduct formed with water. The bimolecular dissociation occurs for the adducts formed with water and MU. In this process, the entity that finally accepts the proton is a cluster of two molecules of the base. We conclude that only one molecule of water is not able to accept the proton donated by the photoacid, a cluster of two molecules of water being required. This cluster is formed in two consecutive steps. First the adduct between the photoacid and one molecule of water is formed, subsequently followed by the reaction of the adduct with a second molecule of water.