Tandem mass spectrometry based collision experiments and computational chemistry (CBS-QB3/APNO methods) indicate that the elusive carbodiimide ion HN=C=NH center dot+ is a stable species in the gas-phase. The ion is the most stable of the family of CH2N2 center dot+ 2 ions and a very high barrier (87 kcal mol (1)) separates it from its tautomer ionized cyanamide, H2N-C N center dot+. The computations also predict that, in the presence of a single H2O molecule as the catalyst, the cyanamide ion isomerizes into the carbodiimide ion. Experiments on the ion-molecule reaction of H2N-C N center dot+ and H2O, a reaction of potential interest in astrochemistry, confirm this prediction. (C) 2009 Elsevier B. V. All rights reserved.