Ionization dissociation of methanesulfonic acid (MSA) in the clusters MSA-NH3-(H2O)(n), n = 0-4, is investigated by quantum chemical calculations. The MSA-NH3 unit without water or with one water molecule is hydrogen-bonded. In the presence of two or more water molecules, it converts to the NH4+center dot CH3SO3- ion pair as a result of direct and spontaneous proton transfer from MSA to NH3. All of the clusters are shown thermodynamically stable in the gas phase with respect to separate monomers. Harmonic frequencies of the clusters confirm the transition from hydrogen-bonded MSA-NH3 to the ion pair as the water molecules are introduced. (c) 2007 Elsevier B.V. All rights reserved.