The reaction between thiourea and μ-superoxo-bis[pentaamminecobalt(III)] pentachloride monohydrate, [(NH3)5Co(O2)Co(NH3)5]Cl5.H2O, hereafter referred to as (I), has been studied in aqueous perchloric acid at [H+] = 0.04 mol dm-3, μ = 0.25 mol dm-3 (NaClO4), and T = 31.0 ± 1.0oC, the reaction conforms to an overall equation: (NH3)5Co(O2)Co(NH3)5 5+ + (NH2)2C=S + 5H2O 2Co2+ + (NH2)2C=O + 2H+ + 10NH3 + SO4 2- The experimental data are consistent with a second-order rate law: -d[[Co(O2)Co5+]/dt = k2[Co(O2)Co5+][TU] with k2 = (7.21 ± 0.09) x 10-2 dm3 mol-1 s-1 . Added NO3 - and CH3COO- catalyzed the reaction. The rate of reaction was dependent on variation in the ionic strength of the reaction medium in the range 0.1 ≤ μ ≤ 0.5 mol dm-3, but display a variety of [H+] dependences in the acid range 0.01 ≤ [H+] ≤ 0.22 mol dm-3. The absence of both kinetic and spectroscopic evidence of complex formation suggests that the reaction proceeds by the outer-sphere mechanism. This deduction is further supported by the non-conformity of the rate data to the Michaelis-Mentens equation.