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Abstract: The reactions of CH3 +, CH2Cl+ and CHCl2 + with standard reference bases were examined in a dual cell Fourier transform mass spectrometer. Deprotonation of methyl cation occurs rapidly via a spin forbidden process to afford triplet methylene even when the reaction is nearly thermoneutral. Bracketing results enable us to assign PA(CHCl)=209.7±2.2kcalmol−1 and PA(CCl2)=205.2±1.9kcalmol−1, the latter value of which is considerably larger than previous determinations. The resulting heats of formation, however, are in good accord with other measurements and lead to recommended values of (CHCl)=74.6±2.4kcalmol−1 and (CCl2)=53.0±2.6kcalmol−1. These values correspond to a 2–3kcalmol−1 lowering of these quantities and are within 1–2kcalmol−1 of high level G3 and W1 predictions based upon the atomization energies and a series of four isodesmic reactions. [Copyright &y& Elsevier]