The measurement of trace amounts of water in process gases is of paramount importance to a number of manufacturing processes. Water is considered to be one of the most difficult impurities to remove from gas supply systems and there is strong evidence that the presence of water contamination in semiconductor gases has a measurable impact on the quality and performance of devices. Consequently, semiconductor manufacturers are constantly reducing target levels of water in purge and process gases. As the purity of gases improves, the problem of quantifying contamination and ensuring that the gases are within specification at the point of use becomes more challenging. There are several established techniques for detecting trace water vapour in process gases. These include instruments based on the chilled mirror principle which measures the dew-point of the gas and the quartz crystal adsorption principle which measures the adsorption of water vapour into a crystal with a hygroscopic coating. Most recently, spectroscopic instruments such as those employing cavity ring-down spectroscopy (CRDS) have become available. The calibration of such instruments is a difficult exercise because of the very limited availability of accurate water vapour standards. This CCQM key comparison aims to assess the analytical capabilities of laboratories for measuring the composition of 10 μmol mol^-1 water vapour in nitrogen. KEY WORDS FOR SEARCH Metrology, traceability, water vapour, process gas, advance manufacturing Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).