The gas phase absolute UV absorption cross section at 213.9 nm for CH3OOH, H2O2, and D2O2 have been determined to be (22.51¿0.78)¿ 10-20, (33.04¿2.17)¿10-20, and (35.37¿4.25)¿1020 cm2 molecule-1, respectively, at 297¿1 K. The corresponding values at 253.7 nm for H2O2 and D2O2 were (7.37¿0.31)¿10-20 and (7.86¿1.04)¿10-20 cm2 molecule-1. The UV absorbance of a slowly flowing mixture of peroxide and helium carrier gas in a Pyrex absorption cell was measured, followed by collection and titration of the eluting peroxide with Fe2+ or I- solutions. The relative cross sections for CH3OOH and H2O2 at 298.1, 326.1, 340.4, and 361.1 nm were also determined using the absolute values at 213.9 nm. The absorption spectra of these peroxides were recorded in the wavelength region between 210 and 365 nm, using a diode array spectrometer, and normalized to the above cross sections to obtain absolute values for the UV absorption cross sections over this entire wavelength region. The obtained results are compared to thos from previous investigations. The atmospheric relevance of our measured cross sections is discussed. ¿ American Geophysical Union 1989