A visible-light activatable photoacid has been studied, which upon irradiation, changes from a weak acid, with a pK(a) of 7.8, to a strong acid, which achieves nearly complete proton dissociation. This process is reversible and the half-life of the proton-dissociation state is ~70s. The long lifetime of the proton-dissociation state is due to a sequential intramolecular photochromic reaction. Using this photoacid, a pH change of 2.2 units has been achieved. In addition, we demonstrated that the photoinduced proton concentration can catalyze an esterification reaction, and greatly alter the volume of a pH-sensitive polymer. This work shows that acid-catalyzed and pH-sensitive processes can be photochemically controlled by using this type of photoacid.