The photoelectrochemical properties of poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonate) in contact with an electrolytic solution containing a redox couple were studied using the theories for the semiconductor-electrolyte interface. When this polymer-electrolyte interface is illuminated with h nu > Eg (gap energy) it exhibits cathodic photocurrent typical of p-type semiconductors, and the flat band potential, density of majority carriers, and the depletion layer thickness can be determined. To complete the band energy diagram of this polymer-electrolyte interface we obtained the band gap energy through the absorption and photocurrent spectra. The relatively low band gap energy (1.5 eV) and the photoeffects observed at the interface suggest its use as the absorbing material in photoelectrochemical cells.