The transient intermediates in the nanosecond laser flash photolysis of ketoprofen, an aryl propionic acid, show the formation of a carbanion in aqueous solutions at pH 7.1. This carbanion incorporates spectroscopic properties from both a ketyl radical anion and a benzylic radical. The ketoprofen carboxylate undergoes biphotonic photoionization, a process that contributes less than 10% to its photodecomposition and leads to a benzylic-type radical after decarboxylation with a rate constant greater than or equal to 1 x 10(7) s(-1). On the other hand, the carbanion forms monophotonically and the unsuccessful attempts to sensitize the formation of the ketoprofen triplet excited state in aqueous solutions suggest that the carbanion precursor is either an excited singlet state or an extremely short-lived triplet. In organic solvents of lower polarity, the excited triplet state is readily detectable.