Theoretical studies of the H2O.O2 complex have been carried out over the past decade, but the complex has not previously been experimentally identified. We have assigned IR vibrations from an H2O.O2 complex in an inert rare gas matrix. This identification is based upon theoretical calculations and concentration dependent behavior of absorption bands observed upon co-deposition of H2O and O2 in argon matrixes at 11.5 +/- 0.5 K. To aid assignment, we have used a harmonically coupled anharmonic oscillator local mode model with an ab initio calculated dipole moment function to calculate the OH-stretching and HOH-bending frequencies and intensities in the complex. The high abundance of H2O and O2 makes the H2O.O2 complex likely to be significant in atmospheric and astrophysical chemistry.