The keto-enol tautomerism of benzoylacetone (1-phenyl-1,3-butadione) in aqueous acidic solution at 25 degrees C was investigated by studying the influence of anionic, cationic, and nonionic surfactants in the UV-vis spectra of benzoylacetone. The percentage of the enol tautomer was found to increase abruptly at surfactant concentration above the critical micelle concentration (cmc). A model is proposed to quantitatively interpret the experimental results, which allows for the determination of the enolization constant in water, K-E, and in micellar, K-E(m), pseudophases; the binding constant of the enol tautomer to micelles, K-s; and the extinction coefficients of the ketonic and enolic forms, epsilon, of benzoylacetone. The formation of the enolate in the absence and presence of micelles in basic media was also studied. The acidity constants of the enolic, K-a(E), and ketonic, K-a(K), forms of benzoylacetone have been determined.