The gas-phase negative-ion chemistry of diazomethane has been investigated by using the flowing afterglow and selected ion flow tube techniques. The gas-phase acidity of diazomethane has been determined to be ΔG°acid(CH2N2) = 365 ± 3 kcal/mol [ΔH°acid(CH2N2) = 373 ± 3 kcal/mol] and the electron affinity of the diazomethyl radical has been estimated to be 2.0 ± 0.3 eV. The reactions of the diazomethyl anion with CS2, COS, C02, and S02and with a series of α,β-unsaturated aldehydes and ketones have been studied. Exothermic addition reactions lead to energetic product ions, which fragment by loss of nitrogen. The resulting carbene anions undergo unusual further reactions, either internally or by reaction with the neutral reagents. Michael addition of the diazomethyl anion to the unsaturated aldehydes and ketones can also result in cyclization and loss of an aldehyde to form a pyrazole anion. Diazomethane can also be reduced by hydride transfer to produce an M + 1 ion, which is most likely the CH2NNH-anion.