Several independent determinations of the pK(a) values of transresveratrol in water have led to conflicting results. Singular value decomposition analysis of UV absorption spectra of trans-resveratrol (t-Resv) in N-2-outgased aqueous solutions buffered to pH values in the 7.0-13.6 range yielded the UV spectra of the three anionic forms and the corresponding pK(a) values: pK(a1), = 9.1(6), pK(a2) = 9.7(7), and pK(a3) = 10.5(5) in very good agreement with calculated theoretical values. The analysis of the absorption spectra guided the assignment of the fluorescence spectrum of each anionic form. With the resolved spectra on hand, we applied the Forster equation to estimate pK(a)* values of 2.5 and 0, respectively, for the p- and m-OH substituents of t-Resv in S-1. Theory supports a proposed mechanism for the reaction of t-Resv anions with O-2.