The PbI2PbO system was examined using thermal analyses (differential thermal analysis and thermogravimetric analysis), differential scanning calorimetry and X-ray difractometry. The experimental procedure was modified to eliminate or minimize the effect of PbI2 volatilization. The condensed phase diagram shows invariant states consistent with oxyiodide compounds with peritectics and eutectics. The conspicuous absence of undetermined liquidus lines suggests glass formation or partially miscible liquid phases. © 1984.