Mechanisms of XN3 (X: F, Cl, Br, I) dissociations are proposed based on B3LYP calculated potential energy surfaces. The energy gaps between the ground-state reactants XN3((x) over tilde (1)A') and the intersystem crossing (ISC) points are only a little lower than respective potential energy barriers of the spin-allowed reactions, XN3((x) over tilde (1)A') --> XN (a (1)Delta) + N-2((x) over tilde (1)Sigma(g)(+)). The ISC point, therefore, is considered as a 'transition state' of the spin-forbidden reactions, XN3((x) over tilde (1)A') --> XN((x) over tilde (3)Sigma(-)) + N-2((x) over tilde (1)Sigma(g)(+)). The methods of IRC and topological analysis of electron density are used to predict the thermal dissociation pathway of the reactions studied. (C) 2003 Elsevier B.V. All rights reserved.