The decomposition mechanism of the pentanitrogen (N-5, N-5(-), N-5(+)) and hexanitrogen (N-6, N-6(+), N-6(-)) clusters and their ions was studied using ab initio MO calculations up to the CCSD(T) level with 6-311+G(3dp) and aug-cc-pVTZ basis sets and also DFT/B3LYP method. While the cyclic anion N-5(-) and open-chain cation N-5(+) are relatively stable with respect to elimination of N-2, the neutral N-5 radical does not exist as a discrete species. The elimination of two N-2 molecules in the neutral N-6 is found to be a concerted process, whereas that in the ionized N-6(+) cluster is stepwise due to the existence of a N-4(+) ion. The fragmentation of a N-6 cluster into two azide (N-3) fragments is, irrespective of its charge state, also st facile process, making them, at most, metastable molecules. Different thermochemical parameters of these N-n species have also been evaluated. (C) 2001 Elsevier Science B.V. All rights reserved.