Photosystem II (PS II)-enriched membranes from spinach chloroplasts were immobilized behind a dialysis membrane on a carbon paste electrode containing 2-6-dimethylbenzoquinone (DMQ). The PS-II-membrane-coated electrode produced an anodic steady-state current under illumination that was attributable to a PS II reaction with DMQ as an electron-transfer mediator between the PS II and the electrode. The magnitude of the steady-state photocurrent depended upon the applied potential, the intensity of the illumination, the amount of DMQ mixed in the carbon paste electrode and the amount of immobilized PS II membranes. The DMQ-mediated photoelectrochemical behavior could be described by an equation of the steady-state kinetics of the PS-II reaction linked to the electrode reaction of DMQ. 2-Methylbenzoquinone, tetramethylbenzoquinone and phenylbenzoquinone, which are sparingly soluble or practically insoluble in aqueous solution, also worked as the mediators when mixed in the carbon paste electrodes.