The optimized structure, rotational constants, vibrational frequencies, changes in spin density, and binding energy are calculated for HOSO2-H2O using ab initio molecular methods. Structurally, the OH bond of HOSO2 increases in length upon formation of the complex, and the SO bond shortens. The OH stretching frequency of HOSO2 is red-shifted 561 cm(-1) from its isolated monomer, and the intensity is increased 10-fold. The initial spin density of the separate monomers is rearranged into a ring pattern surrounding the ring structure of the complex. The complex is bound by 10.6 kcal mol(-1).