Ab initio quantum mechanical methods have been applied to the H5O2+ system, for which experiments are beginning to appear. These methods include basis sets up to triple-zeta plus double polarization plus f functions (TZ2Pf) and levels of correlation up to coupled cluster including single, double, and perturbatively treated connected triple excitations [CCSD(T)]. The potential energy hypersurface is very flat and is sensitive to the level of theory. At the highest level of theory the global minimum is the structure with C-2 symmetry, but the transition state with C-s symmetry is only 0.4 kcal/mol higher in energy. Some low energy excursions into C-1 symmetry are carefully examined. The theoretical dissociation energy for the H3O+-H2O system is predicted to be 33.4 kcal/mol, and is in good agreement with experimental measurements. The harmonic vibrational frequencies and their infrared intensities at several levels of theory are also reported, and compared to the recent spectroscopic observations in Lee’s laboratory.