Ab initio MO calculations have been carried out for the ozone-water 1:1 complexes in order to elucidate the structures and electronic state of the complexes. The QCISD calculations indicated that three structures are obtained as stable forms of O(3)-H(2)O. The most stable structure of O(3)-H(2)O has C(s) symmetry where the central oxygen of O(3) and all atoms of H(2)O are located on the molecular C(s) plane. The dipole of H(2)O orients toward the central oxygen atom of O(3) (i.e., dipole orientation form). The other two forms are cis and trans forms of O(3)-H(2)O where all atoms are located on the molecular plane, and a hydrogen of H(2)O binds to one of the terminal oxygen atoms of O(3) by a hydrogen bond. The binding energies of O(3) to H(2)O for dipole, cis, and trans forms are calculated to be 2.39, 2.27, and 2.30 kcal/mol, respectively, at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311++G((d,p) level. The dipole orientation form is more stable in energy than the cis and trans forms. Rotational constants for the dipole orientation form are calculated to be A = 11.897, B = 4.177, and C = 3.318 GHz which are in good agreement with the experimental values (A = 11.961, B = 4.174, and C = 3.265 GHz). The electronic states of O(3)-H(2)O were discussed on the basis of theoretical results.