The impact of Rossby wave breaking on the subtropical tropopause and intrusions of high potential vorticity air into the tropical upper troposphere (UT) on subtropical humidity is examined using measurements from the Microwave Limb Sounder (MLS) and Atmospheric Infrared Sounder (AIRS) satellite instruments and trajectory-based simulations. Both data sets and the simulations show large gradients in humidity in the vicinity of the intrusions, with very dry air within the intrusions (relative humidity (RH) less than 20%) and very moist air ahead (east) of the intrusions (RH around 80%). The dry air within the intrusion is consistent with air from the lowermost stratosphere, while the increase in RH ahead of the intrusions is consistent with the known deep convection and poleward flow in this region, which rapidly transports moist air from the equatorial lower troposphere into the subtropical UT. As the water vapor content of the ambient air in the subtropical UT is already low, the net impact of intrusions is most likely to increase RH in the subtropical UT.