Following several recent studies of small nitrogen-containing ring compounds for use as fuels, we follow up with calculations of the thermodynamic properties of mixed carbon–boron–nitrogen compounds; specifically, cyclo-CH2BHNH and two isomers of cyclo-C2H4BHNH. The 1,2-isomer of cyclo-C2H4BHNH has the heteroatoms adjacent in the ring, while the 1,3-isomer alternates the boron and nitrogen atoms with carbon atoms in the ring. Our results indicate that the combustion energies are about 41–45 kJ/g, making these compounds among the highest energy density substances we have studied.