High levels of ab initio molecular orbital theory have been used to study the structures, binding energies, vibrational frequencies and equilibrium constants of hydrogen peroxide-water dimers. The geometries of the different possible conformers were optimized at the HF/6-3 1 1 + + G (2d, 2p) level of theory. Five different stationary points have been characterized at this level, but only two are minima. The geometries and vibrational frequencies of these two minima were refined at the MP2/6-3 11 +G( d, p) level. The stretching vibrations of the proton donors show a sizable red-shift. The global minimum corresponds to a tivemembered ring, where both monomers behave simultaneously as proton donors and proton acceptors. In the second minimum, which lies about 2.2 kcal/mol higher, the hydrogen peroxide monomer behaves as a proton acceptor, while water behaves as a proton donor. The binding energies of these two species were obtained at the QCISD (T ) /6-3 1 1 + G( 2d, 2p) level using the MP2 optimized geometries, and the corresponding equilibrium dimerization constants evaluated. A topological analysis of the electronic charge densities of the dimers is presented.