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-H2O. The most stable structure Of O-3-H2O has C, symmetry where the central oxygen Of O-3 and all atoms of H2O are located on the molecular C, plane. The dipole of H2O 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-H2O where all atoms are located on the molecular plane, and a hydrogen of H2O binds to one of the terminal oxygen atoms Of O-3 by a hydrogen bond. The binding energies Of O-3 to H2O 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-H2O were discussed on the basis of theoretical results.