Peroxy radicals were measured using chemical amplification during the four intensive measurement periods of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) campaign from September 1991 to August 1992. One-minute average radical concentrations were obtained with overall uncertainties of 8 to 16 parts per trillion by volume (pptv), depending on the variability of ambient ozone. Thirty-minute averaged concentrations have uncertainties of ¿30%. The peroxy radicals exhibited diurnal behavior expected of an atmospheric species with photochemical origins and correlated with a simple expression that describes radical formation and loss. The application of a steady state analysis to the radical levels suggested that for some conditions, loss processes other than gas phase chemistry may be required to explain the observed concentrations. ¿ American Geophysical Union 1996