A method for manufacturing photocatalytically active titanium dioxide layers on substrate surfaces. The method reduces the effort for the manufacture of photocatalytically active titanium dioxide layers and increases the choice for the coating of suitable substrate materials. In the method, a titanium compound present in the gas phase and water vapor are directed to a preheated substrate by means of gas phase hydrolysis and a titanium dioxide layer is foamed on the surface of the substrate by chemical reaction. In this respect, the titanium compound and water vapor are supplied separately from one another so that a flow speed of at least 0.5 m/s is achieved and the time between the first contact of the two gases up to the impact on the surface of the substrate is kept lower than 0.05 s, and in this process the photocatalytically active titanium dioxide layer is formed on the substrate surface.