Balloon-borne observations of frost-point temperature and ozone in the equatorial western, central and eastern Pacific as well as over equatorial eastern Brazil provide a highly accurate data set of water vapor across the tropical tropopause. Data were obtained at San Crist¿bal, Galapagos, Ecuador (0.9¿S, 89.6¿W), during the late northern winter and the late northern summer in 1998 and 1999 and at Juazeiro do Norte, Brazil (7.2¿S, 39.3¿W), in February and November 1997. Earlier data in the western Pacific region in March 1993 were reanalyzed to extend the scope of the observations. The data show three different circumstances in which saturation or supersaturation occurs and imply different mechanisms for dehydration at the tropical tropopause: (1) convective dehydration, (2) slow-ascent dehydration, and (3) large-scale wave-driven dehydration. Furthermore, air that crosses the tropical tropopause in the late northern summer may be dehydrated further during late northern fall, as the average tropical tropopause rises and cools. Not all soundings show dehydration, and there are clear differences in the frequency and depth of saturation in different regions and seasons. The tropopause transition region can be identified in accurate measurements of relative humidity, even under conditions where ozone observations are ambiguous. Deep convection plays an important role in setting up this transition region, which is then subject to large-scale wave activity and wave breaking at the tropopause or midlatitude intrusions. High relative humidities over regions of strong subsidence show that descending motion in the troposphere is limited to levels below the transition region.