A photoacoustic sensor system for automatic detection of low concentrations of water vapour is described in this paper. A Littman-configuration external-cavity diode laser operating at 1125 nm was used as a light source in combination with a high-sensitivity measuring photoacoustic cell, a reference photoacoustic cell and PC-controlled electronics. The system was calibrated with synthetic gas samples and a detection limit of 13 µmol per mol of water vapour was determined. Adsorption/desorption phenomena at the walls of the measuring photoacoustic cell were found to be an important limiting factor for the sensitivity of the system.