Photoexcitation of acridine or 6-methoxyquinoline to their first electronic singlet state is followed by rapid proton abstraction from water, producing the ion pair PhN*H+ and OH-. During the lifetime of the excited state the ions are effectively separated. By the time PhN*H+ relaxes to the ground state, the solution is poised in acid-base disequilibrium. The transient deviation of the ground-state reactants from equilibrium is monitored through absorption changes of a pH indicator. Under conditions where the indicator is mostly protonated, pH > (In), rapid deprotonation by OH- is recorded. At pH > pK(In), collisional proton transfer, from PhNH+ to In-, predominates and the indicator response is transient protonation. The analysis of the results by numerical solution of the coupled nonlinear differential equation yields the diffusion controlled rate constants of all participating reactions.