A kinetic study on singlet oxygen (O-1(2)) quenching by 4-hydroxycinnamic acid derivatives (ferulic acid (FA) and caffeic acid (CA)) has been performed by measuring time-evolutions of O-1(2) phosphorescence at 1274 nm in ethanol, in an ethanol/acetone mixed solvent and in an ethanol/H2O mixed solvent under acidic, neutral and basic conditions. In organic solvents, the estimated second-order rate constants of O-1(2) quenching (k(Q)) were distributed between 10(5) and 10(6) M-1 s(-1): for example, in ethanol, 2.2 x 10(5) M-1 s(-1) for FA and 5.2 x 10(5) M-1 s(-1) for CA. On the other hand, the k(Q) values in ethanol/H2O solutions were enlarged from those in organic solvents, and varied largely with pH from 10(6) to over 10(8) M-1 s(-1). In the case of FA, the O-1(2) quenching activity decreased in the order of FA(2-) (at pH > 10)>> FA(-) (pH 5-9) > FA (pH < 4). The result suggests that deprotonation due to the acid-base equilibrium affects the O-1(2) quenching activity. For both of FA and CA, the phenolate anion under basic conditions (pH 10-12) showed the largest activity probably because of occurrence of the favored chemical quenching through the electron transfer from the phenolate anion to O-1(2). These behaviors would affect several antioxidant assays in vivo and in vitro targeting O-1(2). (C) 2012 Elsevier B.V. All rights reserved.