(R)-(-)-1,1’-Binaphthyl-2,2’-diyl hydrogen phosphate (BiNPO4H) forms radical anions (lambda(max) = 460 nm and epsilon(460) = 8600 L mol(-1) cm(-1)) on reaction with hydrated electrons. The radical anion undergoes acid-catalyzed protonation with a rate constant of (5.7 +/- 0.9) x 10(9) L mol(-1) s(-1) to form H-adduct, which has absorption at lambda(max) = 410 nm. A similar adduct but having a slightly different absorption lambda(max) = 405 nm (epsilon(405) = 2900 L mol(-1) cm(-1)) is formed on the addition of H atoms to BiNPO4H. Addition of OH. radicals to BiNPO4- also formed an adduct which absorbs at lambda(max) = 405 nm (epsilon(405) = 3100 L mol(-1) cm(-1)). Oxidation of BiNPO4- with strong oxidizing radicals such as SO4.-, Tl2+ and Cl-2(.-) led to the formation of the radical cation. It is also formed by dehydroxylation of the OH-adduct in acidic solution. The radical cation has a broad absorption spectrum with peaks at 340, 405, 460, and 630 nm (epsilon(630) = 2000 L mol(-1) cm(-1)). It decays with a rate constant of 5 x 10(4) s(-1). Reduction potentials of the radical cation and the anion of BiNPO4- have been determined to be 1.70 and -1.51 V vs SHE at pH 9, respectively. The rate constants for electron transfer reactions of these radicals with a number of compounds have been determined as a function of their reduction potentials. The bimolecular rate constants obtained were fitted with the rates predicted by the classical Marcus electron transfer theory. A good free energy gap correlation (log k vs -Delta G) was obtained for the rate constants of reaction of -.BiNPO4- with organic compounds. The rate constants of reactions of +.BiNPO4- with inorganic anions also showed a good free energy gap correlation. But rate constants showed poor correlation with the nucleophilicity of the anion. This indicates that reaction between +.BiNPO4- and inorganic anions is due to electron transfer and not a nucleophilic addition. This was further confirmed from the transient spectrum of the oxidized products formed in the reaction. In contrast to the reaction of the inorganic anions, the rate constants for reaction of +.BiNPO4- with unsaturated compounds followed the rates predicted by Marcus theory only in the region of -Delta G > 0, and in the -Delta G < 0 region the observed rates are higher than the predicted rates and the deviation from the predicted rate increases with decrease in -Delta G. This indicates that when the electron transfer reaction becomes endothermic, addition of the radical cation to the unsaturated compounds increasingly becomes the dominant pathway of the reaction.