Ab initio molecular orbital calculations have been performed to investigate the structures and quantum effects of the proton motion in NH3:HCl:(H2O)(n) (n = 0-3) clusters using a MP2/aug-cc-pVDZ level of theory. Three new stable structures and one transition-state structure are investigated for these clusters. The detailed analyses of the intermolecular interactions suggest that three-body interactions play an important role to determine the relative stability in each size of cluster. The quantum effects of the proton motion result in frequency shifts for proton-stretching modes. Our one-dimensional and two-dimensional models fairly closely reproduce the experimental proton-stretching vibrational frequency of the NH3:HCl cluster. The most stable isomer for n = 1 has a proton-transfer structure, which is weakened by the quantum effects of the proton motion.