alpha-Mn2O3 is of interest as a low-cost and environmentally benign electrocatalyst for the Oxygen Evolution Reaction (OER) in the process of water splitting. Mechanically stable alpha-Mn2O3 electrodes are prepared by annealing of galvanostatically deposited MnOOHx layers on F: SnO2-coated glass. The overpotential eta to achieve a current density of j = 10 mAcm(-2) decreases from 590 to 340 mV with increasing layer thickness. Differential capacitance measurements reveal that this high OER activity can be attributed to the large electrochemically active surface area (ECSA), which scales linearly with the thickness of these highly porous and electrolyte-permeable films. The oxide layers exhibit a reversible oxidation behavior from Mn(III). Mn(IV). Although the intrinsic activity is small compared to that of other OER catalysts, such as NiFeOx, the combination of high ECSA and good electrical conductivity of these alpha-Mn2O3 films ensures that high OER activities can be obtained. The films are found to be stable for > 2 h in alkaline conditions, as long as the potential does not exceed the corrosion potential of 1.7 V vs. RHE. These findings show that alpha-Mn2O3 is a promising OER catalyst for water splitting devices.