The photo-induced anodic STM currents measured with water photo-oxidizing RuS 2 single crystals (100) were found to be strongly dependent on the relative H 2 O humidity and thus on the presence of adsorbed water molecules. Since humidity-dependent currents and photocurrents were not found with highly oriented pyrolithic graphite (HOPG) and have not been reported for other semiconductors, the phenomenon seems to be related to the semiconductor's ability to photoreact with water. This hypothesis was confirmed by verifying a similar effect with (oxygen evolving) TiO 2 . The conclusion is reached that adsorbed water species, which capture photogenerated holes from the semiconductor, are acting as efficient traps for electrons provided by the tunneling tip. The surface states involved can be detected by STM spectroscopy and their peaks estimated to be situated approximately 0.5 V above the valence band edge. Their concentration can be reversibly controlled by the degree of humidity. The described phenomenon is of interest for the understanding of STM currents and may additionally provide a new experimental approach to interfaces which are catalytic for water oxidation.