Increasing mobility of InP films with usage time of one PH3 cylinder prompted an investigation into factors influencing the purity of delivered PH3. The presence of hygroscopic HxPOy residues in a delivery system greatly increases the dry-down time compared to that of a clean system. Static delivery system tests show increasing H2O concentration with time and twice the increase in PH3 versus N-2 over 48 h indicating reaction of metal oxides in components with PH3 to generate H2O. Gas purity may also vary during cylinder usage. Depletion of a high-purity PH3 cylinder shows consistently low gas phase H2O levels before phase-break but increasing levels after phase-break, as the cylinder depressurizes. The results highlight the importance of using pure PH3, employing rigorous cycle-purging procedures to prevent HxPOy contamination, switching out cylinders in good time and using purification technology to control H2O. (C) 2009 Elsevier B.V. All rights reserved.