We have measured the rate constants and product branching fractions for the proton hydrate clusters H3O+(H2O)(n) (n = 0-4), reacting with dimethyl sulfoxide. The reactions were studied over the temperature range 198-298 K. Larger clusters were only studied at low temperatures since they are unstable at higher temperatures. All reactions were fast, approaching the collisional limit. The small clusters are slightly slower than the collisional limit, especially at 298 K. The product distributions were measured only at 198 K. Ail reactions proceeded by formation of H+(CH3SOCH3)(H2O)(n). The major product was often the channel producing two neutral H2O molecules. The product distributions for the larger clusters were probably effected by thermal decomposition as evidenced by production of endothermic products.