Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H center dot(H2O)(n) (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA center dot(H2O)(3), the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (> 10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA center dot(H2O)(5), the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA center dot(H2O)(n) clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.