We have utilized cavity ring-down spectroscopy (CRDS) to characterize the pressure and carrier gas effects on the shape of a water absorption line. The half-width at half-maximum pressure-broadening coefficient for the water line at 10687.36 cm(-1) was measured to be 58 +/- 6 MHz kPa(-1) (0.20 +/- 0.02 cm(-1) atm(-1)) for water in phosphine (PH3). This value is twice as large as the pressure-broadening coefficient for the same absorption transition in the case of water in air. Strong interference from neighboring PH3 lines limits the sensitivity of the system in the region of this absorption line. We have also characterized the spectral neighborhood of several other water absorption transitions for the water-PH3 system: these additional water-PH3 CRDS spectra are presented. We have identified the spectral region around the water line at 10667.76 cm(-1) as optimal for CRDS measurements of the water-PH3 system. Minimal interference from adjacent PH3 absorption transitions in this region enables high-sensitivity, real-time measurements of trace water in bulk PH;. Utilizing this water line, our CRDS apparatus has an estimated detection limit of 50 nmol mol(-1) HO in PH3. (C) 2003 Elsevier B.V. All rights reserved.