The oxidation of ABTS(2)-[2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate)] with a superoxorhodium(III) complex, L-2(H2O)RhOO2+ (L-2 = meso-hexamethylcyclam) is characterized by an acid-dependent equilibrium constant, log(K-e/[H+]) = 4.91 +/- 0.10 in the pH range of 4.89-6.49. This equilibrium constant was used to calculate the reduction potential for the L-2(H2O)RhOO2+/L-2 (H2O)RhOOH2+ couple, E-0 = 0.97 V vs NHE. The pH dependence of the kinetics of the L-2(H2O)RhOOH2+/I- reaction yielded the acid dissociation constant for the coordinated water in L-2(H2O)-RhOOH2+ , pK(a) = 6.9. Spectrophotometric pH titrations provided pK(a) = 6.6 for the superoxo complex, L-2(H2O)-RhOO2+. The combination of the two pK(a) values with the reduction potential measured in acidic solutions yielded the reduction potential E-0 = 0.95 V for the L-2(HO)RhOO+/L-2(HO)RhOOH+ couple. Thermochemical calculations yielded the bond-dissociation free energy of the L-2(H2O)RhOO-H2+ bond as 315 kJ/mol at 298 K.