proves a high reaction rate of electrons with the H + ions to form hydrogen H 2 . Fig. shows hydrogen bubbles on the surface of the photoelectrode, formed during illumination with simulated solar light (AM 1.5). The evolved H 2 gas was collected and analyzed by mass spectrometry. It can be seen from Fig. that it contains only H 2 . The sharp peak of the H 2 curve was induced after a strong pressing of the syringe piston. The absence of an H 2 Se signal proves that the WSe 2 electrode was not corroded during the hydrogen evolution process. The evolved gas volumes of H 2 and O 2 show a linear dependence on the gas evolution time for a constant photocurrent. The ratio of the evolved gas volumes V H2 /V O2 is equal to 2.2, which confirms, within the measurement accuracy of about 10 %, the expected ratio 2. This ratio is due to the fact that the H 2 formation is a two-electron process while the O 2 evolution is a four-electron process. 1