The oxidation of ascorbic acid by thioureapentacyanoferrate(III) has been investigated over the pH range 0.8–7.2 at an ionic strength of 1.0 M (NaClO4). The variations in the rates of oxidation with hydrogen ion concentration are consistent with a reaction scheme involving three pH-related species; ascorbic acid (H2A) (k1 = 0.111 M−1 s−1, ΔH1≠ = 5.1 kcal mol−1, ΔS≠ = −46 cal deg−1 mol−1), the ascorbic anion, (HA−) (k2 = 66 M−1s−1, ΔH2≠ = 5.4 kcal mol−1, ΔS2≠ = −32 cal deg−1 mol−1) and the ascorbic dianion, (A2−) (k3 = 8.78 × 107 M−1 s−1, ΔH3≠ = 3.1 kcal mol−1, ΔS3≠ = −12 cal deg−1 mol−1). The low reduction potential of Fe(CN)5TU2− (E0 = 0.35 V) permits the measurement of the rate of oxidation of the ascorbate dianion, not previously investigated with metal complexes. The second acid dissociation constant of ascorbic acid [Formula: see text] has been measured in 1.0 M NaClO4 over the temperature range studied. A relationship between the rate of oxidation and the reduction potentials of the ascorbic acid species is shown.