Water vapour plays an important role for the chemistry and dynamics of the atmosphere. It is a strong greenhouse gas in the troposphere and contributes to cooling in the stratosphere. As the main source of chemically active HOx radicals, it is linked to many photo-chemical cycles controlling the composition of the middle atmosphere. In order to improve our knowledge of the amount and variability of water in the middle atmosphere, the Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001, observes several thermal emission lines of water vapour in the 486–581 GHz spectral range from the Earth's limb. Bands centred at 488.9 and 490.4 GHz are used to study water vapour and its isotopes, on the basis of four observation days per month. Vertical profiles of View the MathML source, View the MathML source, and HDO are retrieved between roughly 20 and 70 km in the stratosphere and mesosphere. A strong water vapour line at 556.9 GHz is simultaneously measured in a second band, providing information in the mesosphere and lower thermosphere between about 40 and 100 km. Measurements of View the MathML source at 552.0 GHz in monthly intervals complete the picture of middle atmospheric water vapour provided by Odin/SMR. The measurements of the isotope HDO in the 20–70 km altitude range allow to study the isotopic ratio of deuterium in stratospheric water vapour (D/H), potentially supplying information on the origin of stratospheric water vapour: transport of tropospheric air through the tropical tropopause layer (TTL) versus in situ chemical production such as from methane oxidation. The unique measurements of the molecules View the MathML source and View the MathML source containing heavy isotopes of oxygen may provide a crucial test for our understanding of the complex chemical reaction mechanisms controlling the exchange of oxygen between water vapour and ozone.