As complex transition-metal oxides of perovskite structures, many cobaltites are active electrocatalysts promoting oxygen evolution reaction (OER) during electrochemical water splitting. To unveil specific structure-activity relationships for electrocatalytic performance, innovative types of catalysts are required to overcome the inherent high complexity of regular powder catalysts, where thin-film technology gained significance in recent years. As we demonstrate, epitaxial La 0.6 Sr 0.4 CoO 3 (LSCO) thin films can be deposited with controlled bulk properties, surface structure, and stoichiometry on orthorhombic (110) NdGaO 3 single-crystalline substrates by pulsed-laser deposition, providing ideal model systems for this purpose. The epitaxial thin films are dense and single crystalline with sub-nanometer surface roughness and grow well oriented toward the pseudocubic  direction. The LSCO thin films show high activity catalyzing the OER and can carry significant current density loads exceeding 100 mA/cm 2 . Using these model catalysts, X-ray photoemission spectroscopy reveals the degradation of the material under these dynamic conditions, involving cation leaching and a phase transformation of the oxide. An altered surface stoichiometry as well as cobalt hydroxide formation is observed. Our results show that epitaxial model systems can be operated at large current density loads, allowing a systematic study of catalysts and their degradation under highly dynamic conditions.