In an aqueous acetate buffer solution, γ-glutamylcysteine (GGC), which is a precursor to glutathione, acts as a reducing agent and reduces the bridging superoxide in [(NH3)5CoIII(O2)CoIII(NH3)5]5+ (1). This reduction results in the formation of the corresponding peroxide in the complex, [(NH3)5CoIII(O2H)CoIII(NH3)5]5+ (2). During this process, the reductant (GGC) is oxidized and forms its disulfide form. The complex 2 that is formed subsequently undergoes rapid decomposition, leading to the formation of the final products such as CoII, NH3, and O2. Complex 2 undergoes this decomposition process rather than reacting with a second molecule of GGC. When there is an excess of reductants compared to complex 1, the reaction follows first-order kinetics and shows an inverse dependence on proton concentration. Additionally, the observed rate constant (ko) values decrease as the ionic strength (I) of the medium increases. These observations suggest that thiolate anions are the reactive forms of the reductant in the reaction. The pKa value of GGC (1.91) also supports this thiolate ion formation in the experimental pH range