It is shown that use of a balanced detector based on common mode rejection improves the line shape of rovibrational transitions of water vapour considerably. The line shape can be fitted by using Voigt profile. The residuals can be reduced by using collisionally narrowed Galatry profile, but the Dicke narrowing conditions are not satisfied for water vapour in this region of spectroscopy. In such cases, the improvement of fit may become a mathematical artifice without any useful information. Ar-broadened line shape parameters of three water vapour transitions of the 2 1 1 overtone band in the 12150 cm(-1) region are presented. 2004 Elsevier B.V. All rights reserved.