The heats of formation of mono-halogen-substituted singlet carbenes have been determined to be 157 +/- 18 kJ mol(-1) ((X) over tilde (1)A’ CHF), 317 +/- 20 kJ mol-1 ((X) over tilde (1)A’ CHCl), 373 +/- 18 kJ mol-1 ((X) over tilde (1)A’ CHBr), and 428 +/- 21 kJ mol(-1) ((X) over tilde (1)A’ or (a) over tilde (1)A’ CHI). These Delta H degrees(f) values are based upon the following gas-phase acidities, Delta H degrees(acid), of the (.)CH(2)X radicals : 1668 +/- kJ mol(-1) ((CH2F)-C-.), 1610 +/- kJ mol(-1) ((CH2Cl)-C-.), 1593 +/- 8 kJ mol(-1) ((CH2Br)-C-.), and 1566 +/- 11 kJ mol(-1) ((CH2I)-C-.). The acidities were determined by observing the occurrence/nonoccurrence of proton transfer in the reactions of the CHX(.-) carbene radical anions with selected acids in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The Delta H degrees(acid) values reveal that (i) the acidity increases in the series (CH2F)-C-., (CH2Cl)-C-., (CH2Br)-C-., and (CH2I)-C-., i.e. the acidity increases as the halogen atom becomes heavier as observed also for the corresponding halogen-substituted methanes, and (ii) a given (.)CH(2)X radical is approximate to 50 kJ mol(-1) more acidic in the gas phase than the related CH(3)X molecule. The reaction of a given CHX(.-) ion with the parent compound yields X(-) ions in part by a direct S(N)2 substitution and in part by initial proton transfer leading to a [(.)CH(2)X + (-)CH(2)X]* complex, which then reacts to form X(-). In addition to overall proton transfer, the reaction of the carbene radical anions with aliphatic alcohols yields X(-) ions by a process which involves initial proton transfer followed by nucleophilic attack of the RO(-) ion on the generated (.)CH(2)X radical.