Activation parameters are reported for additions of phenylchlorocarbene (PhCCl), pentafluorophenylchlorocarbene (F5-PhCCl), and 3,5-dinitrophenyl-chlorocarbene (3,5-DN-PhCCl) to tetramethylethylene (TME), cyclohexene, and 1-hexene. The order of activation enthalpies is F5-PhCCl > PhCCl > 3,5-DN-PhCCl. Activation enthalpies also increase as the degree of alkene alkylation decreases. In general, the entropies of activation increase in tandem with the enthalpies of activation.