The important gas-phase reaction of H2O and SO3 to give H2SO4 was investigated at the RMP4SDQ/6-311 +G(2df,p) //RMP2(fu)/6-31+GT* + ZPE(RMP2(fu)/6-31G*) level. While the computed overall reaction energy of -20.5 kcal/mol agrees with the experimentally derived value for Delta H-R degrees, = -22.1 +/- 2.3 kcal/mol, both the computed complexation energy (-7.9 kcal/mol to give the SO3.H2O adduct) and the reaction activation energy (19.4 kcal/mol) are very different from earlier results. For T = 298 K, free enthalpies of 0.0 (SO3.H2O), +28.6 (TS), and -10.7 kcal/mol (H2SO4) relative to isolated H2O and SO3 are derived. A new energy decomposition procedure based on a simple point charge model was used together with NBO analysis to examine the bonding in the SO3.H2O complex. Electrostatic attraction of the water dipole to SO3 is as important as charge transfer interaction. However, the changes in electron distribution between the SO3.H2O complex and separated SO3 and H2O are due mainly to polarization, e.g. dipole induction. The calculated and measured frequencies for the complex compare reasonably well.