The effects of metallic cobalt (Co) and cobalt monoxide (CoO), as additives in positive electrodes, on the electrochemical performance of nickel/metal hydride (Ni/MH) power batteries are studied. Commercial Co and CoO are charged at 50°C in 6M KOH solution. The oxidation mechanism of cobalt materials is investigated by observing structural and morphological evolutions during charging. A pure Co3O4-type phase is formed when the starting material is CoO. When Co is used, a cobalt oxyhydroxide (CoOOH) phase is present, together with a tricobalt tetroxide (Co3O4) phase. In both cases, the cobalt concentration in the electrolyte decreases during oxidation. The final product is dependent on the solubility of cobalt and the kinetics of the reaction that consumes cobalt tetrahydroxide [Co(OH)4]2−. The highly compact CoOOH phase, which works well between the nickel foam frame and nickel hydroxide [Ni(OH)2] particles, enhances the power performance of Ni/MH power battery. The Co3O4 phase, which works well in connecting Ni(OH)2 particles, improves the capacitive performance of Ni/MH power battery.