Results are presented for measurements of the concentration of OH performed aboard the NASA P3-B aircraft using the Selected Ion Chemical Ionization Mass Spectrometry technique during the Pacific Exploratory Mission-Tropics (PEM) study. Typical midday boundary layer OH concentrations of 6--8¿106 molecules cm-3 were observed. Diurnally averaged boundary layer OH values obtained near Christmas Island, Kiribati, combined with SO2 and filter measurements of non-sea-salt sulfate reveal that only 12% of the sulfate found in the particulate form can be attributed to gas phase H2SO4 production/condensation. Measurements performed over clouds yielded 2--3 times larger than those typically observed under similar cloudless conditions. ¿ 1999 American Geophysical Union