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Layers of amorphous manganese oxides were directly formed on the surfaces of different carbon materials by exposing the carbon to aqueous solutions of permanganate (MnO₄^- ) followed by sintering at 100-400 °C. During electrochemical measurements in neutral aqueous buffer, nearly all of the MnO_x /C electrodes show significant oxidation currents at potentials relevant for the oxygen evolution reaction (OER). However, by combining electrolysis with product detection by using mass spectrometry, it was found that these currents were only strictly linked to water oxidation if MnO_x was deposited on graphitic carbon materials (faradaic O₂ yields >90 %). On the contrary, supports containing sp³ -C were found to be unsuitable as the OER is accompanied by carbon corrosion to CO₂ . Thus, choosing the "right" carbon material is crucial for the preparation of stable and efficient MnO_x /C anodes for water oxidation catalysis. For MnO_x on graphitic substrates, current densities of >1 mA cm^-2 at η=540 mV could be maintained for at least 16 h of continuous operation at pH 7 (very good values for electrodes containing only abundant elements such as C, O, and Mn) and post-operando measurements proved the integrity of both the catalyst coating and the underlying carbon at OER conditions.