Most chemists agree that the bond dissociation energy of a pi bond (DJ is less than that of a corresponding sigma bond (D"). That is, of the two bonds in A=B the first is weaker than the second. By contrast, there are rather different views on how to estimate /)".Bond dissociation energies are defined for a process in which the broken bond(s), the involved molecules and their states are specified at 25°C (1). Now consider ethylene which has received the most varied-treatment. By definition, its bond dissociation energy or "double bond energy," i.e., D(H2C=CH2), refers to process (la).