The molecular structure and thermodynamic stability of C-nitroso and dinitroso (azodioxide) compounds (CNC) have been studied by using high-level, composite, ab initio method (G4MP2) via the series of appropriate homodesmotic, dimerization reactions. The calculated reaction enthalpies and Gibbs free energies allowed us to estimate relative stability of dinitroso moiety and the role of substituents in thermodynamic stabilization of dinitroso group versus its monomeric precursor. The stability of dinitroso compounds is generally low as is indicated by large positive values for standard enthalpies of formation. The stabilization of the dinitroso group is much more pronounced in alkyl, alkenyl, or alkynyl derivatives than in the aromatic derivatives. The thermodynamic stability of E-stereoisomers of dinitroso compounds is generally larger than their Z-stereoisomer analogues. However, the difference in E/Z stereoisomer stability is quite small in aromatic dinitroso compounds. We have discussed the influence of substituents on the molecular geometry of the nitroso and dinitroso groups. We have also discussed the nature and strengths of solid-state forces pertaining to CNC.