Three-body nonadditive effects in the Ar2O- trimer have been calculated ab initio by means of the fourth-order Moller-Plesset perturbation theory, the coupled cluster theory with the single, double, and noniterative triple excitations, and the intermolecular Moller-Plesset perturbation approach. The genuine three-body effects have been separated from the two-body orientational nonadditivity. The latter arises in this system due to the rotation of the singly occupied p-symmetry orbital from the Sigma and Pi symmetries in the dimer to the A(1), B-1, and B-2 symmetries of the trimer. It is responsible for the collinear form of the trimer. In general, the total three-body effect has been found to be determined by the induction nonadditivity, but the exchange effects become important in the short range. Several models of the total three-body effect have been proposed and fitted to ab initio calculations. (C) 2003 American Institute of Physics.