In situ measurements of [OH], [HO_2] (square brackets denote species concentrations), and other chemical species were made in the tropical upper troposphere (TUT). [OH] showed a robust correlation with solar zenith angle. Beyond this dependence, however, [OH] did not correlate to its primary source, the product of [O_3] and [H_2O] ([O_3]•[H_2O]), or its sink [NO_y]. This suggests that [OH] is heavily buffered in the TUT. One important exception to this result is found in regions with very low [O_3], [NO], and [NO_y]. Under these conditions, [OH] is highly suppressed, pointing to the critical role of NO in sustaining OH in the TUT and the possibility of low [OH] over the western Pacific warm pool due to strong marine convections bringing NO-poor air to the TUT. In contrast to [OH], [HO_x] ([OH] + [HO_2]) correlated reasonably well with [O_3]•[H_2O]/[NO_y], suggesting that [O_3]•[H_2O] and [NO_y] are the significant source and sink, respectively, of [HO_x].