A variety of high-level ab initio techniques [including G2(QCI) and CBS-APNO] have been employed to characterize the various isomers of neutral and protonated C2N2. Notable findings of this study are the following : first, that the calculated proton affinity of NCCN (655 kJ mol(-1)) is approximately 20 kT mol(-1) below the accepted experimental value, and second that the respective proton affinities of the two terminal atoms of cyanogen cyanide, CNCN, are virtually identical (similar to 692 kT mol(-1)). The barrier to unimolecular isomerization between HCNCN+ and CNCNH+ is large; thus, protonation of CNCN at room temperature should generate these two species, as distinct isomers, in comparable amounts. In consequence, we anticipate that the investigation of CNCN protonation presents a considerable experimental challenge.