AbstractA description is given of an excited state photoelectrochemical cell in which the excited state, Ru(bpy)2+*3 (bpy is 2,2′‐bipyridine) is quenched via irreversible electron transfer to Co(NH35Cl+2 which gives Ru(bpy)3+3. The oxidizing equivalents in Ru(bpy)3+3 are transferred via an external circuit to a second half‐cell in which OH− is oxidized to O2. The variation of photogenerated charge and limiting currents with changes in [Co(NH3)5Cl+2], time, and other variables are presented in the context of a kinetic model for the cell. Conditions are also described for the operation of the cell as an actinometer which is useful for the wavelength range 360 nm ≦ λ ≦ 580 nm and exhibits reproducibility comparable to that observed with conventional chemical actinometers.