The invention relates to a photocatalyst containing titanium dioxide, to a method for using it and to its application. A sulphurous titanium dioxide hydrate precipitate is precipitated from an acid titanium oxysulphate solution at a temperature below the boiling point of the solution, e.g. in the range from 70 to 100.degree. C., using crystal nuclei and without addition of base. The precipitate is separated, washed and calcinated. The photocatalytic titanium dioxide thus obtained has a specific area in the range from 100 to 250 m.sup.2/g and a 0.3 to 5% sulphur concentration. Catalytic activity has been confirmed in asetal dehyde decomposition and in anionic (SNC.sup.-).sub.2 radical formation.