CdSe-coated electrodes, formed by electrodeposition of CdSe barrier layers on metallic Ti or porous TiO2 substrates, were characterized by electrochemical impedance spectroscopy in a (photo)cell using aqueous redox electrolytes based on the sulfide/polysulfide or ferro/ferricyanide couples. The influence of electrode material properties, electrolyte contact, thermal annealing, and measurement conditions (illumination, frequency, potential-scan speed) on the shape and features of Mott–Schottky plots was investigated. The obtained information was evaluated on the basis of the ideal Schottky diode model and photocurrent voltammetry data. Deviations from linear diode behavior and uncertainties in the determination of energetic parameters were examined and attributed to the presence of donor density gradients and surface states in the semiconductor electrode, further complicated by chemical corrosion. The origin of the observed non-idealities is inquired into, and specific aspects of the measuring procedure related to the non-stationary character of the interface are discussed.