Understanding underlying processes behind the performance of multilayer photoelectrochemical (PEC) devices is crucial to their continued development. As an operando technique, electrochemical impedance spectroscopy (EIS) is uniquely positioned to detect processes not only within individual layers, but also interactions between the layers themselves during device operation. These devices commonly employ amorphous TiO2 (a-TiO2) as a protection layer against corrosion, because many established and emerging semiconductors themselves are unstable during photoelectrocatalysis. Here, we resolve the appropriate model to describe operating photocathodes performing the hydrogen evolution reaction (HER) using EIS, and propose the origin of a low frequency process previously observed in the literature. As illustrated in Figure , processes in PEC devices are either electronic or ionic in nature. Within the solid state layer(s), electronic processes such as charge generation, recombination and carrier diffusion occur, which influence parameters such as the onset potential and photocurrent density. Ionic processes within solids, such as intercalation and ion migration, can also be present. Within the electrolyte solution, electrochemical reactions, ionic diffusion and reorganization of the Helmholtz layer can occur. The timescales of electronic properties generally range from femtoseconds to microseconds, while those of ionic processes range from microseconds to tens of seconds. The ability to observe these processes using EIS depends on the timescale of the process. Using common instrumentation, EIS detects processes occurring between microseconds and seconds.