Oxygen evolution from water by use of earth-abundant element-based catalysts is crucial for mass solar fuel production. In this report, a mesoporous cobalt oxide with an ultrahigh surface area (up to 250 m(2).g(-1)) has been fabricated through Mg substitution in the mesoporous Co3O4 spinel, followed by a Mg-selective leaching process. Approximately a third of Mg cations were removed in the leaching process, resulting in a highly porous cobalt oxide with a significant amount of defects in the spinel structure. The activated mesoporous cobalt oxide exhibited high oxygen evolution activities in both the visible-light-driven [Ru(bpy)(3)](2+)-persulfate system and the Ce4+/Ce3+ chemical water oxidation system. Under a strong acidic environment, a high turnover frequency (TOF) of similar to 22 X 10(-3) s(-1) per Co atom was achieved, which is more than twice the TOF of traditional hard-templated, mesoporous Co3O4.