AbstractHeats of formation of solid, liquid, and gaseous nitroalkanes have been shown mostly to obey group additivity. Group values have been obtained for carbon atoms attached to one, two, and three nitro groups. The heat of formation of 1,1,1,3,5,5,5,‐heptanitropentane, either solid or liquid, cannot be fitted to the scheme, even allowing for gauche effects. The differences between observed and estimated values for 1,1,1‐fluorodinitroalkanes and 1,2‐dinitroethane are larger than expected and should be further investigated. Activation energies have been calculated for decomposition by five‐center elimination of HONO from mononitro‐ and dinitroalkanes using thermochemistry and estimated activation energies for the reverse reactions. The key data for these estimates were previously reported activation energies for the decomposition of nitroethane and 1,2‐dinitropropane. The calculations also gave values for the heats of formation (in kcal/mole) of nitroethylene 12.4, and 1‐nitropropylene 5.6, and 2‐nitropropylene 1.6. Activation energies were calculated for the competing unimolecular reaction, CN bond fission, from thermochemistry and previously reported activation energies for the decomposition of 1,1‐ and 2,2‐dinitropropane. Comparison of Arrhenius parameters for the two competing processes, namely, HONO and CN bond fission, shows that, for the geminate dinitroethanes and dinitropropanes, CN bond fission is faster about 370°K and, for the mononitroalkanes and for all the mononitroalkanes and dinitroalkanes, CN bond f