Density functional molecular orbital theory was used at the B3LYP/6-311++G(2d,2p)//B3LYP/6311++G(2d,2p) level of theory to study the hydrates of sulfuric acid (H2SO4. nH(2)O) for n = 0-7 and the dimer of the trihydrate of sulfuric acid, (H2SO4. 3H(2)O)(2). Six neutrals of the first six H2SO4. nH(2)O clusters (n = 1-6) were determined to be hydrogen-bonded molecular complexes of H2SO4 and H2O and contain no H3O+ ions. An H3O+ ion and HSO4- formed in clusters of H2SO4. nH(2)O for n = 3-7. This is in contrast to the bulk phase where H2SO4 and two H2O＇s have been converted to two H3O+ ions and a free SO42-ion. The energetics of the hydration reactions also were obtained. The free energy of formation from free H2SO4 and H2O had a minimum at n = 3-4 at room temperature. However, the free energy of formation of H2SO4. nH(2)O from free H2SO4 and H2O decreased rapidly with temperature with the minimum becoming less pronounced and disappearing completely by 248 K. Below 248 K there is no free energy barrier to the formation of larger hydrates.