Ionic dissociations of the halosulfonic acids HSO3X, X = F, Cl, Br in an aqueous environment are studied by density functional theory and ab initio method calculations of the water clusters HSO3X-(H2O)(n),n = 1-4. With a minimum of n = 3, HSO3X is dissociated as a result of proton transfer from HSO3X to H2O. Two competing stable configurations are reported: one with the halogen pointing away from water and the other with the halogen participating in hydrogen bonding with water. In the former, the S-X bond is stable with the successive addition of water molecules while in the latter the bond length increases gradually. (c) 2006 Elsevier B.V. All rights reserved.