Thermodynamic properties of nitroguanidine, hexahydro-1,3,5-trinitro-1,3,5-triazine and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine in the gas phase have been estimated using additivity techniques and statistical calculations. Ideal gas thermodynamic properties of urea, guanidine, methanimine, 2-propanimine, nitramide, N-methyl-N-nitromethanamine and 1,3,5-triazine required for these estimations were calculated by rigid-rotor harmonic-oscillator approximation using the molecular constants available from the literature. Estimations obtained by the difference method in addition to statistical calculations give satisfactory values of thermodynamic properties.