Reaction cross sections for the reactions of protonated water clusters, H+(H2O)(n) (n = 1-6) with dirnethylsulfoxide (DMSO) were measured at the collision energy of 0.1 eV. The cross section at n = 2, which was found to be twice as large as that at n = 4, showed a possibility of direct proton transfer process. The proton is likely to transfer directly from H+(H2O)(2) to DMSO without forming an intermediate complex because of the larger proton affinity of DMSO than that of water dimer. (C) 2003 Elsevier B.V. All rights reserved.