The Raman spectrum of Cs2O2 excited at 632.8 nm shows a single O2−2 fundamental band at wavenumber of 742.5 cm−1 and second and third overtones at 1468.5 and 2176.8 cm−1, respectively. The harmonic wavenumber ωe = 759.5 ± 0.7 cm−1 and anharmonicity constant ωexe = 8.4 ± 0.3 cm−1 were calculated directly from the measurements. These results are compared with ωe = 1151.0 ± 1.5 cm−1 and ωexe = 9.6 ± 0.5 cm−1 recently found for O2− ion in CsO2 by others. The lower ωexe of O2−2 relative to that of O2− indicates a broader (smaller curvature) anharmonic oscillator potential well. Copyright © 2002 John Wiley & Sons, Ltd.