At the 2nd Workshop of the Atmospheric Chemistry Validation of Envisat (ACVE-2), re-processed water vapour data from MIPAS (V 4.61) were evaluated using data assimilation models. The following groups participated: (i) the Data Assimilation Research Centre in collaboration with the Met Office, both UK; (ii) the European Centre for Medium-range Weather Forecasts; and (iii) the Belgian Institute for Space Aeronomy. This paper discusses issues concerning the assimilation of water vapour in the troposphere and stratosphere, and presents preliminary results from the evaluation of the re-processed MIPAS water vapour data. This evaluation exercise was done under the auspices of the Modelling and Assimilation (MA) validation subgroup of the Atmospheric Chemistry Validation Team (ACVT) for Envisat.