(N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr(+)CCl BF4-, 4) is generated by laser flash photolysis (LFP) of the corresponding diazirine (5) and reacted with tetramethylethylene, cyclohexene, 1-hexene, 2-ethyl-1-butene, methyl acrylate, and acrylonitrile. Absolute rate constants are measured for these carbene-alkene addition reactions, and activation parameters are obtained for additions of MePyr(+)CCl BF4- to tetramethylethylene, cyclohexene, and 1-hexene. MePyr(+)CCl BF4- is computed to be a highly reactive, electrophilic, singlet carbene, and experiments are in accord with expectations. Its activation parameters are compared with those of CF3CCl, CCl2, CClF, and CF2. In all cases, enthalpy-entropy compensation is observed, with Delta H double dagger and Delta S double dagger decreasing in tandem as carbenic stability decreases. A qualitative explanation is offered for this phenomenon.