In situ observations of OH and HO_2 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 HO_x in the lower stratosphere (tropopause to ~21 km) and upper troposphere (~10 km to tropopause). These measurements span a latitude range from 70°S to 90°N and a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of the observed [HO_2]/[OH] to changes in temperature, [O_3], [CO], [NO], [ClO], and [BrO] is investigated. The measured ratio is accurately described (~±10%) by a steady-state model constrained by the measured mixing ratios of O_3, CO, NO, ClO, and BrO, where the model is valid for conditions of HO_x cycling much faster than HO_x production and loss. The concentration of HO_2 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 HO_2 is controlled by the local chemistry between the HO_x radicals and O_3, CO, NO, ClO, and BrO. The response of [HO_x] to changes in [NO_x] and [O_3] is demonstrated. Further observations are necessary to illustrate the response of HOx to changes in halogen concentrations. A quantitative understanding of [HO_2]/[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.