Cyanogen N-selenide radical cations, NCCNSe.+ (4(.+)), have been produced by dissociative ionization of 3,4-dicyano-1,2,5-selenadiazole (3). By using a new hybrid tandem mass spectrometer, it is shown that these ions are excellent agents of selenation of nitriles and isonitriles, producing nitrile N-selenide ions (RCNSe.+) and isoselenocyanate ions (RNCSe.+), respectively. The connectivities of these ions have been established by collisional activation, and the stabilities of the corresponding neutrals were probed by neutralization-reionization experiments and ab initio G2(MP2,SVP) calculations. The neutral nitrile N-selenides are found by experiment and theory to be observable species in the gas phase. All the cations are potential energy minima, and their calculated fragmentation energies are in accord with experimental observations. NCCNSe.+ is found to be an effective Se.+ transfer agent because of its low N-Se bond dissociation energy and exceptional low-lying LUMO. The effect of substituents (CH3, NH2, OH, F, Cl, Br, I, CN, and SCH3) on the structures and stabilities of RCNSe neutrals is also reported.