In situ observations of OH and HO2 from the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric TRacers of Atmospheric Transport (STRAT), and Polar Ozone Loss in the Arctic Region in Summer (POLARIS) NASA ER-2 field campaigns are used to examine the partitioning of HOchi in the lower stratosphere (tropopause to similar to 21 km) and upper troposphere (similar to 10 km to tropopause). These measurements span a latitude range from 70 degreesS to 90 degreesN and a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of the observed [HO2]/[OH] to changes in temperature, [O-3], [CO], [NO], [CIO], and [BrO] is investigated. The measured ratio is accurately described (similar to+/-10%) by a steady-state model constrained by the measured mixing ratios of O-3, CO, NO, CIO, and BrO, where the model is valid for conditions of HOchi, cycling much faster than HOchi production and loss. The concentration of HO2 depends on [OH], which, to first order, has been observed to be a simple function of the solar zenith angle in the lower stratosphere.(1) The partitioning between OH and HO2 is controlled by the local chemistry between the HOchi radicals and O-3, CO, NO, CIO, and BrO. The response of [HOchi] to changes in [NOchi] and [O-3] is demonstrated. Further observations are necessary to illustrate the response of HOchi to changes in halogen concentrations. A quantitative understanding of [HO2]/ [OH] is important, since many of the reactions that control this ratio are directly involved in catalytic removal of O-3 in the lower stratosphere and production of O-3 in the upper troposphere.