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 () 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 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 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). ¿ 2001 American Geophysical Union