In order to determine the lowest energy isomer of the hydroperoxyl radical dimer, H2O4, ab initio quantum mechanical methods were employed to predict the geometrical structures, relative energies, harmonic vibrational frequencies, and associated IR intensities of both open chain and cyclic isomers. Two minima were located on the open chain potential energy surface, one of C-2 symmetry and one of C-1 symmetry. The relative energies of the different H2O4 structures vary strongly with level of theory. The most reliable treatment used in the present study predicted that the global minimum is the closed-shell C-1 chain isomer which is lower in energy than the planar C-2h triplet cyclic isomer by 1.6 kcal mol(-1) including zero point vibrational energy corrections. It is argued that both structures should be observable, depending on the method of preparation.