Reactivities of the structural isomers CCN+ and CNC+ were examined in a selected-ion flow tube at 300 ± 5 K. The less reactive CNC+ isomer was identified as the product of the reactions of C+ + HCN and C+ + C2N2; in these reactions only CNC+ can be produced because of energy constraints. Rate coefficients and branching ratios are reported for the reactions of each isomer with H2, CH4, NH3, H2O, C2H2, HCN, N2, O2, N2O, and CO2. Ab initio calculations are presented for CCN+ and CNC+; a saddle point for the reaction CCN+ → CNC+ is calculated to be 195 kJ mol-1 above CNC+. The results provide evidence that the more reactive CCN+ isomer is unlikely to be present in measurable densities in interstellar clouds. © 1988, American Chemical Society. All rights reserved.