Abstract: The assumption that the bond energies in the free radicals are equal to ones in the parent molecules is the reason for bringing on two sets of pi-bond energy data. If the assumption is not accepted, the same values of pi-bond energies can be estimated from thermochemical and kinetic data. The theoretical base and the error range for Benson-O'Neal empirical rule for estimating activation energies of thermal cis-trans isomerization have been discussed.