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A fast perturbation in proton concentration can be induced in aqueous solution using a pulsed ultraviolet laser and suitable photolabile compounds which, upon photoexcitation, irreversibly release protons. The volume change and the rate constant for the reaction of the photodetached protons with proton-accepting groups in solution can be monitored using time resolved photoacoustics. A typical proton concentration jump of 1 μM can be obtained with a 200-μJ laser pulse at 308 nm. Reaction dynamics from 20 ns to 5 μs can be easily followed. The methodology we establish represents a direct, time-resolved measurement of the reaction volume in proton transfer processes and an extension to the nanosecond–microsecond range of traditional relaxation techniques, such as stopped-flow. We report example applications to reactions involving simple molecules and polypeptides.