Sub-ppm hydrogen chloride (HCl) and water vapour (H2O) monitoring using photoacoustic spectroscopy in optical fibre manufacturing is reported. The development and performance of a sensor based on an acoustic resonant configuration is described, and on-site measurements are presented. Two DFB lasers emitting in the 1370 nm and 1740 nm range were used for the detection of H2O and HCl, respectively. A detection limit (defined for a SNR=3) of 60 ppb for HCl and 40 ppb for H2O was achieved. Contamination sources of the carrier gas used for the fibre preform manufacturing are identified and discussed. © Springer-Verlag 2006.