Reaction of excited ketoprofen (KP) with tryptophan (Trp) and tyrosine (Tyr) in a phosphate buffer solution was studied by the transient absorption spectroscopy. Both amino acids, which would interact with KP in bovine serum albumin [Monti, S. [2009] Phys. Chem. Chem. Phys., 11, 9104-9113], accelerated the proton transfer reaction to yield 3-ethylbenzophenone ketyl biradical (EBPH) from KP carbanion, which was produced by photoexcitation of KP- through decarboxylation. By means of the actinometry method with benzophenone, the reaction quantum yield was successfully estimated to be fairly large, and Trp, Tyr, DOPA and 4-methylphenol were found to be a good proton donor for the carbanion. The formation rate constants of EBPH by the amino acids (k(r)) were also determined to be (2.7 +/- 0.1)x10(9)M(-1)s(-1) for Trp and (7.8 +/- 0.4)x10(8)M(-1)s(-1) for Tyr, which were larger than those by basic amino acids and dipeptides reported. The reason for the highly efficient proton transfer reaction with Trp and Tyr would be explained by difference of the activation energy for the reaction. These results suggest that the proton transfer should be a key process for an initial photoreaction of KP with a protein, causing photosensitization in vivo.