. Can. J. Chem. 55, 3581 (1977).The mechanisms of oxidation of the three di-hydroxy substrates in the title reactions have been investigated in acid media [HCIO,] = 0.04-1.00 M, at an ionic strength of 1.0 M [LiCIO,] over the temperature range 5-35°C using the stopped-flow method. In contrast to the reactions of hydroquinone (k2(25"C) = 1.43 x 10, M -' s-' , AH* = 3.6 + 0.4 kcal mol-I, AS* = -34 + 8 cal deg-' mol-') and catechol (k2(25"C) = 9.5 x 1 0 2 M -' s-', AH* = 5.9 _+ 0.9 kcal mol-', AS* = 27 + 8 cal deg-I mol-I) where no hydrogen ion dependence is observed over the range studied, the rate variations of oxidation of ascorbic acid (H2A) are consistent with two reactions involving H2A (k, = 77.4 M -' s-I, AH* = 4.5 , 0.6 kcal niol-', AS* = -35 + 5 cal deg-I mol-') and HA-( k , = 2.41 x lo5 M -' s-' , AH* = 2.0 + 1 kcal mol-', AS* = -17 + 6 cal deg-I mol-I). The dissociation constant of ascorbic acid has been redetermined over the temperature range studied. Solvation effects are considered important in these outer-sphere systems, which may be discussed in terms of the Marcus relationship. Comparisons with related reactions are discussed.