The triphenylmethane leucohydroxide salt 4-(dimethylamino)-4 '-(trimethylammonium) triphenylmethanol iodide has been used as a photoactivatable caged hydroxide to rapidly increase the pH of a neutral aqueous solution on the nanosecond time scale. Photoexcitation of the compound with a nanosecond ultraviolet laser pulse results in the formation of a colored carbocation with a rate greater than 10(8) s(-1); recombination occurs with a multiexponential kinetics. The dissociation is completely reversible on time scales of 10(2) s and offers a large time window after the pH-jump, in which the proton transfer reactions can be studied. The effectiveness of the photodissociation process to increase the pH of the solution has been demonstrated by means of the pH indicator bromoxylenol blue.