Proton-transfer reactions in water are essential to chemistry and biology. Earlier studies reported on aqueous protontransfer mechanisms by observing light-triggered reactions of strong (photo)acids and weak bases. Similar studies on strong (photo)base−weak acid reactions would also be of interest because earlier theoretical works found evidence for mechanistic differences between aqueous H + and OH − transfer. In this work, we study the reaction of actinoquinol, a water-soluble strong photobase, with the water solvent and the weak acid succinimide. We find that in aqueous solutions containing succinimide, the proton-transfer reaction proceeds via two parallel and competing reaction channels. In the first channel, actinoquinol extracts a proton from water, after which the newly generated hydroxide ion is scavenged by succinimide. In the second channel, succinimide forms a hydrogen-bonded complex with actinoquinol and the proton is transferred directly. Interestingly, we do not observe proton conduction in water-separated actinoquinol−succinimide complexes, which makes the newly studied strong base−weak acid reaction essentially different from previously studied strong acid−weak base reactions.