Ambient measurements of peroxy radical concentrations were made using the chemical amplifier (CA) and determined independently from photostationary state deviations (PSSD), NO2+h&ngr;(+O2)⇄NO+O3, derived from simultaneous measurements of O3, NO, NO2, and jNO2. The data were collected in the Rural Oxidants in the Southern Environment experiment during 19 days of July 1990. A reasonably good correspondence between the two methods is observed for many of the days, although estimates from the PSSD method for some of the days are higher by as much as a factor of 2. Scatter observed between estimates probably results from several sources: uncertainties in the calibration of the RO2-HO2 instrument, rapid changes in the ozone background, rapid alterations in the solar flux induced by intermittent cloud cover, and imprecisions in making simultaneous measurements of , 2>, 3>, and jNO2 required for the PSSD method. Possible origins of bias in the two measurement techniques are discussed. Theoretical estimates of the peroxy radical concentrations were made using the measured suite of trace gas concentrations for 2 days, one for which the CA and PSSD estimates of peroxy radical concentrations differed significantly (July 14) and one for which they showed good agreement (July 11). Theoretical estimates for July 11 checked well with the results from both methods. Those for July 14 fell between the CA and PSSD estimates. These results suggest that the PSSD method may have a bias toward higher estimates on some days and/or that the CA method may have a bias for the lower estimates for reasons which are discussed. ¿ American Geophysical Union 1993