The hydrolysis reaction of sulfur trioxide to form sulfuric acid in small water clusters is investigated using density function theory and ab initio methods. The equilibrium geometries for the reactant clusters that contain SO3 and one to four water molecules, SO3.(H2O)(n), n=1-4, as well as the corresponding transition state and product clusters, are calculated at the levels of B3LYP/6-31+G*, B3LYP/6-311++G**, and MP2/6-311++G**. The relative energies of the reactant, transition state, and product are determined for each of the four clusters. The energy barrier required to form H2SO4 from the reactant cluster is found to decrease sharply with the number of water molecules in the cluster. Most significantly, the SO3.(H2O)(4) cluster is found to be unstable and to form the ion pair HSO4- and H3O+ with little or no energy barrier. This work reveals the possibility of more pathways for the formation of sulfuric acid than have been previously considered.