Silicon-modified titanias prepared by thermal reactions of mixtures of titanium tetraisopropoxide and tetraethyl orthosilicate with various ratios in 1,4-butanediol at 300 degrees C (glycothermal reaction) were calcined at 500 degrees C in air, treated in an NH3 flow at 600 degrees C for 1 h, and then annealed in air at different temperatures. The properties of the thus-obtained nitrified Si-modified titanias were characterized by UV-vis diffuse reflectance spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption, and electron spin resonance. The products had absorptions in the visible-light region and exhibited photocatalytic activities for decomposition of acetaldehyde under visible-light irradiation. It was found that the amount of Si modification significantly affected the amount of doped nitrogen and the visible-light-induced photocatalytic activity. Higher annealing temperature (500 degrees C) decreased the population of oxygen vacancies and increased photocatalytic activity in spite of the fact that the amount of doped nitrogen was reduced by the annealing at that temperature. The Si-modified titania with Si/Ti ratio of 0.3, nitrified at 600 degrees C for I h and annealed at 500 degrees C, exhibited the highest photocatalytic activities.