The purpose of this work was to re-measure the enthalpy Delta(r)H(T)(o) of free-surface vaporization of hexagonal (h) Mg3N2, AlN, GaN, InN and Si3N4 and cubic (c) BN by the third-law method and by comparing them with the enthalpies calculated from thermodynamics to deduce the composition of primary products (bN and cN(c)) for the generalized decomposition reaction: MaNb+2c <-> aM + bN + cN(2). In addition to these nitrides, the literature data were used for the investigation of vaporization of h-Be3N2 and c-AlN and c-GaN. The relative standard deviation in determination of Delta(r)H(T)(o) values is <= 1% and in determination of b/(b + 2c) ratio is better than 2-4%. As a result of this study, some important features in the decomposition mechanism of different nitrides were revealed: the identity in composition of primary products for h-Be3N2 and h-Mg3N2 (b/(b + 2c) = 0.75) and h-AlN and h-GaN (b/(b + 2c) = 0.42) and the release of only molecular nitrogen in cases of c-BN, c-AlN and c-GaN (b/(b + 2c) = 0). The last feature that was observed earlier for cubic metal oxides [B.V. L'vov, V.L. Ugolkov, F.F. Grekov, Kinetics and mechanism of free-surface vaporization of zinc, cadmium and mercury oxides analyzed thermogravimetrically by the third-law method, Thermochim. Acta 411 (2004) 187-193.] supports the effect of crystal symmetry on the decomposition mechanism. The distance between the closest atoms (N-N) in a crystalline lattice has a secondary or no effect on the composition of primary products. These features might be useful for the development/improving of appropriate technologies (including nano-technologies) for production of solid materials by the chemical vapour deposition. (C) 2005 Elsevier B.V. All rights reserved.