Nitrogen-doped (N-doped) TiO2 particles are prepared by plasma treatment using nitrogen gas and thermal stability of visible light activity in the N-doped TiO2 particles is investigated. The particles are heat treated in either nitrogen or air atmospheres in the range of 373 to 673 K. Photocatalytic activity under visible light irradiation in the N-doped TiO2 particles decreases with increasing heat treatment temperature both in nitrogen and air atmospheres and is almost nonexistent at 673 K. Visible light absorption is conspicuous in the N-doped TiO2 particles after heat treatment in nitrogen atmosphere. Doped nitrogen atoms are removed from the particles by the heat treatment to form oxygen vacancies which cause visible light absorption. On the other hand, visible light absorption decreases with heat treatment in air. The doped nitrogen atoms are substituted with oxygen atoms during heat treatment such that the structure reverts to that of the original TiO2 particles. (C) 2008 Elsevier B.V. All rights reserved.