The structure, stability, and reactivity of [NCl3]H+ ions from protonation of gaseous trichloramine NCl3 with CnH5+ (n = 1, 2) and H3O+ ions were studied by mass spectrometric techniques, Fourier transform ion cyclotron resonance (FT-ICR), and triple quadrupole (TQ) mass spectrometry and computational methods. Kinetic experiments, performed to determine the basicity of NCl3, suggested the existence of two [NCl3]H+ populations. Consistent with the higher basicity of the nitrogen atom in NCl3, theoretical calculations characterize the most stable isomer as the H-NCl3+ ion and suggest for the less stable one, the HCl-NCl2+ structure, corresponding to the trichloramine protonated on the chlorine atom. The calculated gas-phase basicities (GB) of NCl3, 679.9 and 692.6 kJ/mol at the B3LYP and CCSD(T) level of theory, respectively, are fully consistent with the experimental value, 687.3 +/- 8 kJ/mol. The proton affinity (PA) of NCl3 is 721.5 +/- 8 kJ/mol. The chloronium ion affinities (Cl+ affinities) of the Cl atom of monochloramine and dichloramine were calculated from the enthalpy of formation (DeltaH(f)degrees) of [H2NClCl+] and [HN(Cl)Cl-2(+)] ions. Whereas the affinity of the nitrogen atom for the H+ and the Cl+ ions and the affinity of the chlorine atom for the Cl+ ion decrease on going from NH2Cl to NCl3, the opposite trend is observed for the proton affinities of the chlorine atom. Studies on the reactivity of [NCl3]H+ ions toward simple nucleophiles showed both a remarkable chlorinating and protonating ability, in accordance with their Lewis and Bronsted acid character.