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Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently …