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.