Abstract: The thermal decomposition of caledonite has been examined by simultaneous differential thermal analysis, thermogravimetry and mass spectrometry. Structural H2O and CO2 are liberated endothermically between 300 and 400°C leaving a residue of lead sulphate, oxysulphate, and Cu(I) and Cu(II) oxides. A series of sharp endothermic peaks between 850 and 950°C correspond to phase transition and melting reactions of the PbO-PbSO4 mixture. The sulphate anion breaks down above 880 °C. Mass spectra of the gaseous decomposition products show SO2, SO, and O2, although SO is an artefact arising from ion fragmentation of the SO2 within the mass spectrometer. The residue at 1060 °C is composed predominantly of 2PbO · PbSO4 and Cu(I) and Cu(II) oxides.