Titanium dioxide powders prepared by a wet method, i.e., the hydrolysis of titanium tetra-isopropoxide or titanium tetrachloride with an aqueous ammonia solution. followed by calcination at temperatures above 330 degrees C, exhibit photocatalytic activity in the visible-light region owing to N doping. The maximum absorption of visible light by the N-doped TiO2 was about 50% at around 440 nm. Thermal analysis revealed that N doping occurs upon the oxidation of ammonia included in titanium hydroxide with lattice oxygen in TiO2. XPS analysis showed that the N doped in TiO2 is less than 1.3 at.% and is not bound directly to Ti. The oxidation state of doped N was found close to that of NO. Quantum yield for the CO photooxidation on the N-doped TiO2 in the visible region was less than that in UV region. These results show that the N doping by the wet method is similar to impurity doping such as metal ion doping. (c) 2005 Elsevier B.V. All rights reserved.