Studying catalysts in situ is of high interest for understanding their surface structure and electronic states in operation. Herein, we present a study of epitaxial manganite perovskite thin films (Pr_1−xCa_xMnO₃) active for the oxygen evolution reaction (OER) from electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES) at the Mn L- and O K-edges, as well as X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states have been performed in ultra-high vacuum and in water vapor under positive applied bias at room temperature. It is shown that under the oxidizing conditions of the OER a reduced Mn²⁺ species is generated at the catalyst surface. The Mn valence shift is accompanied by the formation of surface oxygen vacancies. Annealing of the catalysts in O₂ atmosphere at 120 °C restores the virgin surfaces.