The gas-phase proton-transfer reaction of ammonia-hydrogen chloride and the effect of the first three water molecules are investigated by high-level ab initio calculations on the molecular clusters NH3-HCl-(H2O)(n), n = 0, 1, 2, 3. The equilibrium structures, binding energies, and harmonic frequencies of the clusters as well as the potential energy surfaces along the proton-transfer pathway of ammonia-hydrogen chloride are calculated at the second-order Moller-Plesset perturbation (MP2) level with the extended basis set 6-311++G(d,p). Either without water or with one water molecule, the ammonia-hydrogen chloride system exists as a usual hydrogen-bonded complex. With two or three water molecules, the system becomes an ion pair resulting from the complete transfer of a proton from hydrogen chloride to ammonia. The potential energy surfaces along the proton-transfer pathway are examined to understand the effect of the water molecules. The harmonic frequencies and infrared intensities of the clusters provide additional evidence in support of the transition from the hydrogen bond to the ion pair structure as the water molecules are stepwise introduced. On the basis of these results, we conclude that ammonium chloride might be formed by the gas-phase reaction of hydrogen chloride with ammonia in the presence of adequate water vapor.