The structure and the reactivity of gaseous (NHCl2)H+ ions formed upon CH4 chemical ionization of monochloramine and dichloramine in water were investigated by FT-ICR (Fourier transform ion cyclotron resonance) mass spcctrometry and theoretical methods. Bracketing experiments performed in order to evaluate the basicity of NHCl2 suggested the existence of two (NHCl2)H+ isomers. The basicity computed by MO SCF calculations allowed us to characterize the most stable (NHCl2)H+ isomer as having the H2NCl2+ structure, corresponding to dichloramine protonated on the nitrogen consistent with the higher basicity of this atom than of the chlorine atom of NHCl2 and the less stable isomer as having the H2NCl-Cl+ structure. The gas-phase basicity, CD, of dichloramine was estimated to be 721.4 +/- 10 kJ mol(-1) from bracketing experiments, according to the method based on the relationship between the efficiency of proton transfer and the standard free energy change of the process. This value is consistent with those from calculations at the B3LYP and CCSD(T)/G-311++G(3df,3pd) levels of theory. The proton affinity, PA, of NHCl2 was evaluated to be 757 +/-10 kJ mol(-1). The (NHCl2)(H+) ion behaves as a protonating and chlorinating agent. An excellent correlation exists between the absolute Cli. affinities of ammonia, monochloramine, and dichloramine, estimated 546, 523, and 390 kJ mol(-1) respectively and their proton affinities. The results show that as substitution of H by Cl atoms in NH3 increases, both the PA and Cl+ binding energy, BE, of chloramines decrease, owing to the electron-withdrawing effect of the Cl atoms on the nitrogen.