The infrared spectrum of the hydrogen peroxide dimer has been studied in argon matrices at 17 K. The spectra have been assigned from concentration dependencies, experiments with different H/D ratios, photolysis experiments, and quantum chemical ab initio SCF calculations. The dieter has a cyclic structure with both components acting as hydrogen bond donors and as hydrogen bond acceptors. There is a significant coupling between the two bonded OH stretches in the two molecules and a relatively strong coupling between the corresponding HOO bends.