Photochemical diodes were prepared using a thin film of TiO2 made from titanium isopropoxide by a sol-gel method. A half part of a conductive SnO2 film plated on a quartz plate was coated with the TiO2 film, and the other parts of the SnO2 substrate were coated with a thin Pt film. The photochemical diode thus prepared shows photocatalytic activity for hydrogen evolution from aqueous ethanol solutions containing NaOH or HClO4. No other products are detected in the gas phase. Hydrogen evolution from the Pt part is definitely observed, indicating electron transfer from the TiO2 film to the Pt film through the SnO2 film. Another type of photochemical diode, the TiO2 film coated on a Pt plate, is found to show higher activity than the TiO2/SnO2/Pt device for the hydrogen photo-evolution without any support electrolyte. Hydrogen evolution on this device, however, occurs on the TiO2 side but not on the Pt side. These results are discussed in terms of a photo-electrochemical mechanism at semiconductor surfaces.