Airborne measurements of the hydroxyl radical were performed aboard the NASA P-3B using the Selected Ion Chemical Ionization technique during the Pacific Exploratory Mission (PEM)-Tropics B study. Typical midday boundary layer OH concentrations of 6–7 × 106 molecule cm−3 were observed in the vicinity of Christmas Island (0°-3°N)) and 7–8 × 106 molecule cm−3 in the vicinity of Tahiti (17°-22°S). Photochemical box model calculations of the concentration of OH ([OH]) throughout the entire mission yield generally good agreement (slope = 1.26) with a tendency for the model to slightly overestimate the measured OH. Boundary layer measurements of OH in an air mass containing low ozone concentrations (<5 ppbv) yielded a midday [OH] of ∼5×106 molecule cm−3, some 45% lower than 7×106 molecule cm−3 observed in air masses at the same latitude with more typical ozone concentrations (30–40 ppbv). Measurements performed under clouds yielded [OH] of ∼2.5×106 molecule cm−3, a value which is a factor of 2 lower when compared to data taken on either side of the cloud. This decrease corresponds to the decrease in j(O3) to produce O(1D).