The adsorption and photodecomposition of ethanol that was employed as a model substrate of organic pollutants were investigated in the presence of adsorbed water with TiO2 by, solid-state (HNMR)-H-1 spectroscopy in order to obtain information about the actual photocatalytic reaction sites of organic pollutants on TiO2 photocatalysts. Consequently it was revealed that ethanol molecules in practical conditions as contaminants were firmly captured in mobile physisorbed water layers to be photodecomposed to CO2 and water after photoillumination, suggesting that actual photocatalytic reactions should proceed in physisorbed water layers.