α-SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar-to-hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α-SnWO4/NiOx interface is performed by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2+ to Sn4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo-based photoemission spectra simulation, the presence of a thin oxide layer at the α-SnWO4/NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new-found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid-state interfaces in multi-layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.