The ferromagnetic and antiferromagnetic states of anatase and rutile TiO2 doped with N atoms at oxygen sites were investigated by density functional theory calculations for a number of model N-doped TiO2 structures. The each N dopant at an O site exists as an N2- ion with net spin moment of 1.0 mu(B). In general, the spin exchange coupling between the spin moments of two N dopants is strong when the two dopants are coordinated to a common Ti atom, and whether the coupling between the two N atoms is ferromagnetic or antiferromagnetic in such a case depends on the angle N-Ti-N angle. (C) 2009 Elsevier B.V. All rights reserved.