We present an optimal estimation retrieval for tropospheric H<sub>2</sub>O and δD applying thermal nadir spectra measured by the instrument IASI (Infrared Atmospheric Sounding Interferometer) flown on EUMETSAT's polar orbiter METOP. We document that the IASI spectra allow for retrieving H<sub>2</sub>O profiles between the surface and the upper troposphere as well as middle tropospheric δD values. A theoretical error estimation suggests a precision for H<sub>2</sub>O of better than 35 % in the lower troposphere and of better than 15 % in the middle and upper troposphere, respectively, whereby surface emissivity and atmospheric temperature uncertainties are the leading error sources. For the middle tropospheric δD values we estimate a precision of 15–20‰, with the measurement noise being the dominating error source. We compare our IASI products to a large number of quasi coincident radiosonde in-situ and ground-based FTS (Fourier Transform Spectrometer) remote sensing measurements and find no significant bias between the H<sub>2</sub>O and δD data obtained by the different techniques. Furthermore, the scatter between the different data sets confirms our theoretical precision estimates.