Mechanisms of RN3 (R=CH3, CH3CH2, (CH3)(2)CH, (CH3)(3)C) dissociations are proposed based on CAS, MP2 and B3LYP methods. The energy gaps between the ground-state reactants RN3 and the intersystem crossing (ISC) points are only a little lower than respective potential energy barriers of the spin-allowed reactions, (RN3)-R-1 --> (RN)-R-1 + N-1(2). The ISC point, therefore, is considered as a transition state of the spin-forbidden reactions, (RN3)-R-1, --> (RN)-R-3 + N-1(2). The methods of IRC and topological analysis of electron density are used to explain and predict the thermal dissociation pathways of the reactions studied. (C) 2004 Elsevier B.V. All rights reserved.