Airborne measurements of the hydroxyl radical, OH, performed during the First Aerosol Characterization Experiment (ACE 1), using the Selected Ion Chemical Ionization Mass Spectrometry (SICIMS) technique are presented. Adaptations of the previous ground-based technique for measurement aboard an aircraft platform are discussed, including an inlet for the straightening and slowing of the airflow, calibration, and measurement considerations at changing pressure altitudes. Steady state model calculations of the concentration of OH, , throughout the entire mission were generally in good agreement, with a slight bias toward an overestimate of the measured . The largest discrepancies between measured and modeled values occurred for measurements in the boundary layer, and those in and around clouds, with the model overestimating the by ~40% in the boundary layer or inside clouds, and underestimating it by ~30% near clouds. The low model near clouds can be attributed to underestimating the actinic flux calculated from Eppley radiometer measurements. The model overestimates in the boundary layer and inside clouds may in part be due to a lack of heterogeneous losses of HOx species in the model. Models developed at the National Oceanic and Atmospheric Administration Aeronomy Laboratory and at Georgia Institute of Technology produce similar results with differences being attributed to the methods of calculating photolysis rates. Calculations of high noon for a flight out of Hobart using the O(1D) quantum yields from Talukdar et al. <1998> produce 17% higher values than those calculated using the currently recommended Jet Propulsion Laboratory values. ¿ 1998 American Geophysical Union