Abstract: A method and a device for photocatalytic oxidation of organic substances in air on a photocatalytic surface of semiconductive metal oxide, air containing the organic substances being caused to flow over the photocatalytic surface and the surface being irradiated with activating light. The relative humidity of the air (RHair) and/or the temperature of the photocatalytic surface (Tcat) are regulated so that the combination of R Hair and Tcat is caused to fall within predetermined acceptable combinations of RHair and Tcat to establish and maintain 0.2-8 monolayers of water molecules on the photocatalytic surface.