The photocatalytic decolourisation of two azo dyes-Reactive Red 198 and Direct Green 99-in an aqueous solution by the artificial visible light radiation was investigated. The industrial metatitanic acid (H2TiO3) obtained directly from the sulphate technology installation was N-doped and used as photocatalyst. H2TiO3 Was calcinated at different temperatures, ranging from 300 to 500 degrees C, for 4 or 20 h, respectively, in an ammonia atmosphere. The UV-vis/DR spectra of the modified catalysts exhibited an additional maximum in the vis region (lambda approximate to 476.8 nm, E-G = 2.60 eV for catalysts calcinated for 4 h and lambda approximate to 479.5 nm, E-G = 2.59 eV for catalysts calcinated for 20 h, which may be due to the presence of nitrogen in TiO2 particles. The chemical structure of the modified photocatalysts was investigated using FTIR/DRS spectroscopy and the presence of nitrogen was confirmed. A photocatalytic activity of the investigated catalysts was determined on the basis of a decomposition rate of azo dyes. The decomposition of Reactive Red 99 increased with increasing the calcination temperature of photocatalysts, whereas the activity of the prepared photocatalysts towards Direct Green 198 degradation was as follows: 300-20 h < 40020 h < 500-20 h < 300-4 h < 400-4 h < 500-4 h. Both, the calcination time and temperature had no influence on the amount of nitrogen-doped into TiO2 structure. The inversely proportional linear dependence between the decomposition rates of azo dyes and the intensity of the band attributed to the hydroxyl groups for both dissociated water and molecularly adsorbed water was observed. With increasing temperature of calcinations, the amount of the hydroxyl groups decreased, whereas the decomposition of azo dyes increased. (c) 2005 Elsevier B.V. All rights reserved.