The structure and the reactivity of gaseous NH3Cl+ ions obtained from direct protonation of aqueous monochloramine by CI/CH4 and from ionization of a Cl-2 plasma containing trace amounts of ammonia have been investigated by FT-ICR mass spectrometry. The results characterized the NH3Cl+ ions arising from both experiments as having the NH3-Cl+ structure, consistent with the results of MO SCF calculations pointing to the higher basicity of the nitrogen than the chlorine atom of NH2Cl. The gas-phase basicity of monochloramine has been estimated to be 761 +/- 5 kJ mol(-1) from bracketing experiments according to the procedure based on the relationship between the efficiency and the standard free-energy difference of proton transfer. This value is consistent with those from theoretical calculations at the B3LYP and CCSD(T)/6-311++G(3df,3pd) level. In agreement with the protonation site, the NH3Cl+ ions behave as a protonating and chlorinating agent but addition is also observed.