We present the first simultaneous measurements of HDO and H2O in the tropical upper troposphere (UT) and lower stratosphere (LS) as derived from infrared solar absorption spectra acquired by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. We find, surprisingly, that the observed HDO/H2O ratio does not decrease with altitude in this region despite a factor of 4--5 decrease in the water vapor mixing ratio. This observation is inconsistent with the view that dehydration in the tropical UT/LS is by gradual processes, and suggests a major role by convective processes.