A simple new device configuration for the conversion of optical energy into chemical energy is described. The devices, designated ’’photochemical diodes’’, consist of small sandwichlike semiconductor structures. When immersed in an appropriate electrolyte and exposed to light, such devices cause redox reactions to occur, such as the decomposition of water, and the oxidation of sulfide ions to form sulfur. Certain configurations result in up‐conversion of the incident photon energy and appear to represent an inorganic analog of photosynthesis.