This paper deals with the gas-phase thermodynamic properties of endothermic compounds potentially formed during monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) hypergolic reactivity. The standard enthalpies of formation at 298.15 K are determined by means of quantum chemistry calculations along with protocols developed for these compounds. The resultant data, currently previously unavailable for almost all of these compounds, are potentially critical to the modeling of combustion chemistry of this bipropellant combination.