Water electrolysis to produce ‘green H₂’ with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co₃O₄ with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co₃O₄(111), CoFe₂O₄(111), and Fe₃O₄(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe₂O₄(111) is found to be up to about four times more active than Co₃O₄(111) and about nine times more than Fe₃O₄(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe₂O₄(111), the oxyhydroxide’s Fe/Co concentration ratio is stabilized by partial iron dissolution.