AbstractThe small‐signal a.c. response of photoelectrochemical cells with Nb‐doped TiO2 anodes has been studied for frequencies in the range 10−2‐104 Hz. A non‐linear least‐squares analysis of the complex admittance/impedance data leads to a detailed equivalent circuit for the cell. The analysis and results are compared with a recently reported Relaxation Spectrum Analysis [4]. The equivalent circuit contains passive elements, and a constant‐phase‐angle (CPA) element Q = kα(iω)−α. It is shown that a “distribution of relaxation times” model, and a “universal dielectric response” model describe frequency‐dependent elements, which are of the CPA‐form. The space‐charge capacitance in the present equivalent circuit obeys the Mott‐Schottky relation, while the CPA‐element is attributed to current inhomogeneities. Solid state aspects of H2 reduction provide a rationale for the current inhomogeneity model.