AbstractCuBi₂O₄ has recently emerged as a promising photocathode for photo‐electrochemical (PEC) water splitting. However, its fast degradation under operation currently poses a limit to its application. Here, we report a novel method to study operando the semiconductor‐electrolyte interface during PEC operation by surface‐sensitive high‐energy X‐ray scattering. We find that a fast decrease in the generated photocurrents correlates directly with the formation of a metallic Bi phase. We further show that the slower formation of metallic Cu, as well as the dissolution of the electrode in contact with the electrolyte, further affect the CuBi₂O₄ activity and morphology. Our study provides a comprehensive picture of the degradation mechanisms affecting CuBi₂O₄ electrodes under operation and poses the methodological basis to investigate the photocorrosion processes affecting a wide range of PEC materials.