The appearance potential of the CH2Cl+ ion has been measured from methyl chloride, dichloromethane, bromochloromethane, and ethyl chloride. The results lead to ΔHf(CH2Cl+) = 230 ± 3 kcal/mole. The heat of formation of CHCl2+ is estimated to be 215 ± 3 kcal/mole from the appearance potential of this ion in the mass spectrum of chloroform and of bromodichloromethane. By the indirect mass spectrometric method ΔHf(CH2Cl) = 29 ± 3 kcal/mole and ΔHf(CHCl2) = 15 ± 3 kcal/mole are obtained. The ionization potentials of a number of chlorine-containing compounds have been measured.