Spinel oxides are promising catalysts for oxygen evolution reaction (OER). The catalytic activity can be enhanced by octahedral ions; however, the effects of tetrahedral ions on OER catalysis remain unclear. A study of the effects of tetrahedral ions on OER catalysis was conducted by synthesizing spinel oxides (ZnxCo3-xO4) with various Zn and Co ratios (x = 0-1). Zn0.4Co2.6O4 showed the highest OER activity, although Zn ions have lower catalytic activity than Co ions at both sites. In the structure of ZnxCo3-xO4, it was discovered via Rietveld analysis and X-ray absorption spectroscopy that Zn2+ ions mainly occupy the tetrahedral sites. The highest Zn0.4Co2.6O4 activity is attributed to a moderate amount of cation defects on the surface caused by Zn2+ dissolution; however, excessive Zn2+ ion amounts led to an unstable surface structure. This study elucidates the OER catalysis enhancement attributed to the ion type at tetrahedral sites in spinel oxides. (C) 2020 Elsevier Inc. All rights reserved.