Theoretical studies of the (H2OO2)-O-. 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 (H2OO2)-O-. complex in an inert rare gas matrix. This identification is based upon theoretical calculations and concentration dependent behavior of absorption bands observed upon codeposition of H2O and O-2 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 O-2 makes the (H2OO2)-O-. complex likely to be significant in atmospheric and astrophysical chemistry.