A new approach for the theoretical exploration of the potential energy surface and thermodynamic properties evaluation of structurally-nonrigid molecular complexes explicitly accounting the rovibrational dynamics of monomers within the complex has been developed and applied to the estimation of thermodynamic properties of the hydrogen bonded complexes H2OO3 and H2O-SO2. The new approach consists of the classical consideration of intermolecular motions of monomers, direct ab initio calculation of intermolecular potential, and the Monte Carlo integration during the partition function evaluation. The full six-dimensional intermolecular potential energy surfaces (PES) of these complexes required for the thermodynamic calculations and consisted of 14016 (H2O-O3) and 9672 (H2O-SO2) unique points were calculated by the ab initio (MP2/6-311++G(2d,2p)) method. The local minima found at the calculated PESﾒs were used as starting points for the farther high-level geometry optimization (up to QCISD/aug-ccpVTZ and MP2/aug-ccpVQZ). It was found that the structure of the global minimum of H2O-SO2 complex is in the agreement with the available experimental data whereas the global minimum of H2O-O3 corresponds to the twisted asymmetric hydrogen-bonded structure (C1) different from that one proposed on the basis of microwave spectra and previous quantum chemical calculations. By using the new approach, the thermodynamic properties and the equilibrium constants of H2O-O3, and H2O-SO2 were obtained for the first time with explicit accounting their rovibrational dynamics. The best estimates for the K0(298) of the complexes H2O-O3, and H2O-SO2 are 1.05*10-2 and 3.15*10-2, respectively (empirically corrected values are 8.7*10-3 and 2.5*10-2). The corresponding uncertainty of these values is estimated as 25-35% (based on the known values of (H2O)2 and (D2O)2 equilibrium constants). The results obtained can significantly affect the conclusions made on the basis of many modern atmospheric models. The work was supported by the Russian Foundation for Basic Research (project No. 07-03-00390)