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The spontaneous and CO2-catalyzed decomposition of peroxynitrite yields HO• and -CO3• radicals, respectively, together with •NO2. The geminate HO•/•NO2 and -CO3•/•NO2 pairs undergo competitive in-cage collapse to nitrate and diffusive separation. Free HO• and -CO3• radicals react with H2O2 and, in the presence of O2, suitable alcohols or formate to produce superoxide, which is trapped by the •NO2 to form peroxynitrate. The formation of peroxynitrate may influence the rate of change in optical density at 302 nm, the wavelength normally employed to monitor peroxynitrite decay, leading to misleading kinetic traces. Tetranitromethane (TNM) was used as a colorimetric probe for superoxide to quantify the yield of free HO• (27−28%) and free -CO3• (32−33%). The yields of both of these free radicals are in excellent agreement with other recent estimates. Superoxide was also detected in some oxygenated aldehyde-catalyzed peroxynitrite decompositions both by peroxynitrate formation and by its reaction with TNM. Supe...