Understanding the mechanism of the oxygen evolution reaction (OER) is essential to develop better electrocatalysts for solar fuel generation. Measuring the pH dependence of the OER activity can provide insights on the reaction path that are otherwise difficult to access experimentally, in particular on the coupling of protons and electrons during the reaction. We argue that the use of a pH-dependent reference electrode, such as the reversible hydrogen electrode, is more suitable for these studies as it assures that the overpotential is fixed while varying the pH. We provide criteria for pH dependence when this reference is used and validate the existing results with our measurements on RuO2 powders. A statistical analysis of the existing results allows us to sketch trends in the reaction order on pH with respect to the number of d electrons, oxidation states, and crystal families, providing the groundwork for future OER mechanistic studies on oxides. (C) 2015 Elsevier B.V. All rights reserved.