Organic semiconductors (OSCs) have emerged as promising active layers for photoanodes to drive photoelectrochemical (PEC) oxidation reactions. Interfacing an OSC with an inorganic electron transport layer (ETL) is key to enabling both high performance and stability. While spectroelectrochemical techniques have been established for the evaluation of inorganic interfaces, allowing rational optimization toward higher performances, a similar level of understanding for hybrid organic-inorganic interfaces remains elusive. To close this knowledge gap, we first perform a systematic parameter study (ETL thickness, potential dependency, and light intensity) on a state-of-the-art organic photoanode to establish factors determining the photoelectrochemical impedance spectroscopy (PEIS) response. Coupled with in situ UV-Vis characterizations, key charge transfer processes are clearly assigned to the PEIS features.