Ab initio RHF calculations with the 3–21G basis set have been carried out on cycloadditions of CF2 and CCl2 with ethylene. Although p-complex intermediates are predicted for both reactions at this level, MP2/3-21G calculations imply that there are no complexes in reactions of CCl2 or more reactive carbenes with ethylene or substituted alkenes. Nevertheless, negative activation energies can be observed, since ?G reaches a maximum due to the increase in —T?S for these bimolecular reactions. The apparent “entropy control” for reactive carbenes results from the rapid decrease in ?H. As the reactivity of the alkene increases, the transition state shifts to an earlier point on the free energy surface, where —T?S‡ is lower, but ?H‡ is higher than for less reactive alkenes. Model potentials are developed for ?H and —T?S which reproduce experimental behavior, without the assumption of intermediates.