In situ electrochemical generation of 2-dimensional Co3O4 (2D-Co3O4) nanoplates has been achieved by anodizing CoOx/Co precursors in 1 M KOH solution. X-ray diffraction characterization suggests that the CoOx/Co precursor is oxidized to Co3O4 before the onset potential of oxygen evolution reaction (OER). Scanning electron microscopy shows that instead of retaining its original morphology, the irregular-shaped nanoparticles undergo a morphological transition into hexagonal nanoplates during the oxidation process. The mechanism of the morphological changes is discussed and a two-step topotactic transformation pathway from Co(OH)(2) to CoOOH to Co3O4 is proposed. Amorphization and recrystallization of the catalysts are also found at different stages of the electrochemical measurements. The 2D-Co3O4 nanoplates exhibit high OER catalytic activity. (C) 2015 The Electrochemical Society.