Abstract : The characteristic reactivity of the radical anion OCC(-) has been investigated in the gas phase at 298 K through determination of rate coefficients, products, and branching fractions for each of 29 ion-molecule reactions. A wide variety of reactions is observed including abstraction of H, H(+), and Hsub2(+), nucleophilic displacement, charge transfer, and reactions involving electron detachment. Many of the reactions involve cleavage of the C(- )-CO bond, consistent with the relatively small C(-)-CO bond energy and the proposed electronic structure of the ground state anions in which both radical and charge are centered on the terminal carbon. Similarities are noted between the chemistry of OCC(-) and its neutral analogue OCC and between the chemistry of OCC(-) and the radical anions O(-) and 0-Csub6Hsub4(-). Most reaction products observed are consistent with reaction mechanisms involving initial attack of the terminal carbon in OCC(-) on the neutral reaction partner. The gas-phase acidity of HCCO is bracketed between those of CHsub3NOsub2 and CHsub3CHO, yielding 1502 + or - 8 > delta G deg acid(HCCO) > or = 1463 + or - 8 kJ mol(-1) and 1531 + or - 12 > delta H deg acid(HCCO) > or = 1491 + or - 12 kJ mol(-1). Observation of H atom transfer from CHsub2Clsub2 to OCC(-) indicates that delta H deg f(OCC(-)) > or = 148 + or - 12 kJ mol(-1) and gives a large lower limit of delta H deg acid > or = 1507 + or - 15 kJ mol(-1). These and related thermochemical values, including the hydrogen bond dissociation energy in HCCO, are compared with literature values.