The mechanism for the gas-phase production of particulate ammonium halides is studied by density functional theory and ab initio calculations on the molecular clusters of ammonia-hydrogen halide, (NH3-HX)(n), n=1, 2, 4; X=F, Cl, Br. The equilibrium structures of the clusters demonstrate an abrupt transition from hydrogen-bonding in the single unit of NH3 -HX to the NH4+. . .X- ion-pair interactions in (NH3-HX)4 resulting from spontaneous proton transfer from hydrogen halide to ammonia. The onset of the ion-pair form is at n=2 for HCl and HBr and at n=4 for HF. The ion-pair structure is the stable form of the larger clusters to maximize favorable electrostatic interactions and to closely resemble the crystal structures of ammonium halides. The calculated vibrational frequencies of the clusters provide additional evidence in support of the transition from hydrogen-bonding to ion-pair interactions as cluster size increases. The study shows that it is possible to form particulate ammonium halides directly from pure NH3 and HX vapors in the homogeneous gas phase. (C) 2001 American Institute of Physics.