Under alkaline conditions mixed NiFe oxides/oxyhydroxides are among the most active catalysts for the oxygen evolution reaction (OER). Here we investigate Ni0.5Fe0.5 clusters as a well-defined model highly active electrocatalyst system for the OER. The electrodes were prepared using gas-phase deposition of mixed Ni0.5Fe0.5 metallic clusters produced by cluster beam deposition (CBD), a technique offering precise control of composition and loading. Highly dispersed Ni0.5Fe0.5 clusters were deposited at 1 equivalent monolayer and used as OER catalyst in 1M KOH. The low loading allows assessment of the intrinsic catalytic activity and their extensive structural characterization by XAFS and XPS spectroscopies. Ni0.5Fe0.5 clusters demonstrate high stability as there is no apparent potential increase after a 12 h constant current density anodization test. After electrochemical aging, the lowest overpotential was achieved and amounts to 372 mV at 10 mA cm(-2). The Tafel slope reaches 37 mV dec(-1). The measured electrochemical activity is also compared with other systems. After OER reaction in KOH the cluster surface is enriched in NiOOH, while concomitant Fe depletion from the catalyst surface is observed. These findings help to shed light on the formed active phase at the nanoscale.