Abstract Films of a series of manganese-cobalt mixed valency spinel oxides of theoretical formula Mn xCo 3-xO 4, (x = 0; 0.25; 0.5; 0.75 and 1), have been prepared by spray pyrolysis at 150 °C on conductive glass in order to relate solid state and surface chemistry to electrocatalytical activity towards the oxygen evolution reaction (OER). X-ray diffraction (XRD) and IR spectroscopy characterized their bulk crystalline properties. Both techniques have confirmed that all oxides belong to the spinel family. Their surface properties were characterized by XPS yielding the Co 3+ / Co 2+ surface ratio. The kinetics of the OER was investigated in 1 M KOH by means of steady state current-potential curves, obtained in the potentiostatic mode and by reaction order determination. The order of reaction ranging from 1.0 to 1.2, and the Tafel slopes ranging from 67 to 69 mV dec −1 can be tentatively interpreted by a mechanism involving a Temkin type of electrosorption of OH intermediates associated with an electrochemical rate-determining step. Replacing progressively Co by Mn, the electrodes were increasingly electrocatalytically less active, surface Co 3+ ions being the active sites, and making Co 3O 4 the most active in the series.