The enthalpies and. Gibbs energies of formation are examined for over 100 aliphatic cis- and trans-diazenes using the G3, G3(MP2), and CBS-QB3 model chemistries for compounds ranging from two to ten heavy atoms. The values computed using bond separation reactions were corrected to compensate for the presence of higher energy conformers. Comparison of several homologous and isomeric series shows that alkene thermochemistry is not an adequate model for the diazenes (HN=NH) as the group equivalents of the latter differ significantly from those of the former when they are adjacent or one atom removed from the nitrogen-nitrogen double bond. Methylene insertion adjacent to the double bond is less exothermic for the diazenes than it is for the alkenes whereas the same insertion one carbon atom away from the double bond is more exothermic than that observed for the alkenes. The data argues that molecular modeling programs parametrized using existing experimental data will need to be recalibrated.