The oxygen reduction reaction (ORR) activity of ZrO2 based, carbon-supported nanoparticles is not conclusively reported in literature. This study examines the dependence of the ORR activity on the used precursors as well as on the heat-treatment atmosphere and temperature. We further determine the ORR activation energy and the ORR mechanism. Various precursors containing Zr and/or N were employed in the synthesis, and the ORR activity was measured by rotating (ring) disk electrode (R(R)DE) voltammetry in both acidic and alkaline electrolyte as well as by measurements in a single-cell polymer electrolyte membrane fuel cell (PEMFC) configuration. We show that even the most active ZrO2 based ORR catalysts exhibit an activity gap of ca. two orders of magnitude compared to the DOE target of 300 A/cm(3) for PGM-free ORR catalysts, thus requiring further development. Our RRDE analysis suggests a primarily 2-electron ORR mechanism in the case of the tested catalysts in acid, which in turn provides a consistent temperature dependence between RDE and PEMFC experiments, allowing also for a mechanistic (re-) interpretation of experimental results in the literature. (C) The Author(s) 2016. Published by ECS. All rights reserved.