Interactions and proton transfer reactions of halosulfonic acids (HSO3X, X = F, Cl, Br) with ammonia (NH3) in the hydrated clusters, HSO3X-NH3-(H2O)(n) (n = 0, 1,2,3), are investigated by density functional theory and ab initio molecular orbital theory. The HSO3X-NH3 systems alone are hydrogen-bonded despite of the high acidity of HSO3X, while one water molecule is enough to assist the transfer of a proton from the acid to ammonia. Large hydration energies and favorable free energies are found for each of the first few water molecules, indicating high stabilities of the small hydrated clusters in the gas phase. (c) 2005 Elsevier B.V. All rights reserved.