In this contribution, we demonstrate the presence of high-spin Fe3+ in Fe-substituted ZrO2 (FexZr1-xO2-delta), as deduced from X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and Fe-57 Mossbauer spectroscopy measurements. The activity of this carbon-supported FexZr1-xO2-delta catalyst toward the oxygen reduction reaction (ORR) was examined by both rotating (ring) disk electrode (R(R) DE) method and single-cell proton exchange membrane fuel cells (PEMFCs). DFT calculations suggest that the much higher ORR mass activity of FexZr1-xO2-delta compared to Fe-free ZrO2 is due to the enhanced formation of oxygen vacancies: their formation is favored after Zr4+ substitution with Fe3+ and the oxygen vacancies create potential adsorption sites, which act as active centers for the ORR. H2O and/or H2O2 production observed in RRDE measurements for the Fe0.07Zr0.93O1.97 is also in agreement with the most likely reaction paths from DFT calculations. In addition, Tafel and Arrhenius analyses are performed on Fe0.07Zr0.93O1.97 using both RRDE and PEMFC data at various temperatures. (C) The Author(s) 2019. Published by ECS.