Resonance stabilization energies derived from enthalpies of combustion vary considerably with the choice of reference species. If we assign a reference C-C single bond order (0.8) that is intermediate between trans-1, 3-butadiene (0.8378) and the single bonds in cis-2-butene (0.7679), we obtain resonance stabilization energies in good agreement with the estimates of Dewar. At the other extreme, if we assign a reference C-C single bond order (0.7760) that is intermediate between 1, 3-butadiene twisted 90 • (0.7874) and cis-2-butene (0.7679), we obtain resonance stabilization energies in good agreement with the estimates of Allinger. We prefer the middle ground, using the reference C-C single bond order from 1, 3-butadiene twisted 90 • (0.7874). This species is obviously not an option as a reference for experimentally based estimates of resonance energies, but it provides a single bond between sp 2 hybridized carbons with no possibility of π-delocalization.