Water is a detrimental impurity even at concentrations of 10 nmol/mol or less in source gases for compound semiconductor epitaxial growth. Oxygen complexes from water incorporation cause degraded luminescent efficiency and reduced minority-carrier lifetimes. Most techniques for detecting water in process gases have poor accuracy below 1 mumol/mol and require frequent calibration and control of ambient humidity. Cavity ring-down spectroscopy (CRDS), in contrast, makes use of a fundamental physical property of H2O molecules-the optical absorption line strength-and a time-constant measurement to provide a water concentration value with high precision and low uncertainty even in the nmol/mol range. We describe the CRDS technique and present the first CRDS measurements of trace H2O contamination in unpurified and purified phosphine. We also report secondary-ion mass spectrometry measurements of the O concentration profiles within a multi-layer film grown using molecular-beam epitaxy in which respective film layers were grown with the purified and unpurified phosphine previously characterized by CRDS. (C) 2002 Elsevier Science B.V. All rights reserved.