<p>Single pixel, tropospheric retrievals of HDO and H<sub>2</sub>O concentrations are retrieved from Atmospheric Infrared Sounder (AIRS) radiances using the optimal estimation algorithm developed for the Aura Tropospheric Emission Spectrometer (TES) project. These retrievals are evaluated against co-located TES observations taken between 2006 through 2010. We evaluate the error characteristics and vertical sensitivity of AIRS measurements corresponding to five days of TES data (or 5 global surveys) during the N. Hemisphere summers between 2006 and 2010 (~ 600 co-located comparisons per day). We find that the retrieval characteristics of the AIRS deuterium content measurements have similar vertical resolution and uncertainty in the middle-troposphere as TES but with slightly less sensitivity in the lower-most troposphere, with a typical degrees-of-freedom (DOFS) in the tropics of 1.5. The difference in sensitivity to the lower-most troposphere is mostly likely due to the reduced spectral resolution as previous studies found that spectral resolution was the primary factor, relative to signal-to-noise, affecting the vertical resolution of nadir sounding retrievals (Worden et al., 2004). The calculated measurement uncertainty is ~ 30 per mil (parts per thousand relative to the deuterium composition of ocean water) for a tropospheric average between 750 and 350 hPa, the altitude region where AIRS is most sensitive. Comparison with the TES data suggest that the calculated and actual uncertainty of a single target AIRS HDO/H<sub>2</sub>O measurement ~ 30 per mil. Comparison of AIRS and TES data between 30 S and 50 N suggest that the AIRS data is biased low by ~ −2.6 per mil with a latitudinal variation of ~ 7.8 per mil. This latitudinal variation is consistent with the accuracy of TES data as compared to in situ measurements, suggesting that both AIRS and TES have similar accuracy.</p>