High levels of ab initio molecular orbital theory were used to study the structures, binding energies, vibrational frequencies, and equilibrium constants of hydrogen peroxide dimers. The geometries of the different initial structures considered were optimized at the HF/6-311 + + G(2d,2p) level of theory. Five different stationary points have been characterized at this level, but only two of them were minima. The geometries of these two minima were refined at the MP2/6-311 + G(d,p) level. Their vibrational frequencies, calculated at the same level of theory, show a sizeable redshift of the stretching vibrations of the proton donors. The global minimum corresponds to a six-membered ring of C-i symmetry, while the second minimum is a five-membered ring, which lies about 1.1 kcal mol(-1) above the global one. The formation of the latter implies a considerable enhancement of the dipole moment. The binding energies of these two species were obtained at the QCISD(T)/6-311 + G(2d,p) level using the MP2 optimized geometries. The equilibrium dimerization constants for hydrogen peroxide are considerably smaller than those for water, due to significant entropic effects. A topological analysis of the electronic charge densities of the dimers shows that both cyclic minima present weaker hydrogen bonds than noncyclic dimers.