We present quantum chemical calculations of several stationary points in the SO2 + H2O system. Final calculations at the QCISD(T)/ 6-311++G(d,p)//MP2/6-31G(d) level predict the following gas-phase thermochemistry at 270 K. (1) SO2 + H2O --> H2SO3(Delta H degrees = +44.9 kJ mol(-1); Delta G degrees = +92.7 kJ mol(-1); E-a greater than or equal to 130 kT mol(-1)). (2) SO2 + H2O --> (SO2-H2O) (Delta H degrees = -7.6 kJ mol(-1); Delta G degrees = +20.1 kJ mol(-1); K-p = 1.29 x 10(-4)). The high activation barrier for H2SO3 formation and its large positive Delta G degrees are consistent with this species having never been detected. Lower level calculations on reaction 1 in the aqueous phase indicate a positive Delta G degrees and high activation barrier in solution as well. The calculated 1:1 SO2-water complex geometry, vibrational frequencies, and rotational constants are in good agreement with experimental results. Its low binding energy suggests that it is unimportant in the atmosphere.