We present a theoretical study of the influence of ring strain and conjugation on the reaction energies of NO2 fission from the carbon atom of cyclic nitramines with gem-dinitroalkyl groups. It is found that the relative strain energy between the reactant and product of a reaction largely affects its reaction energy. The influence of conjugation on the reaction energies seems constant for TNAZ, DNNC and PNTZ, which is about 15 kcal mol(-1). Understanding of these effects may help us to design new high-energy density and high-thermal stability explosives. © 2005 Elsevier B.V. All rights reserved.