Summary: The thermal decomposition of portlandite, Ca(OH)₂, has been studied in air and in vacuum using X-ray single-crystal techniques. In air, the crystals were decomposed in situ on the goniometer arcs whilst X-ray reflections were simultaneously recorded. The transformation to CaO was not accompanied by topotaxy; this is attributed to the high nucleation rate of CaO crystallites in air. When Ca(OH)₂ single crystals were decomposed under vacuum (and subsequently exposed to X-rays), some orientation of the CaO crystallites occurred. These results are compared with those of previous workers using electron-diffraction techniques. Decomposition of Ca(OH)₂ single crystals commences at ∼ 450 °C in air and at ∼ 230 °C in a vacuum of 10⁻⁶ mmHg; reaction commences at crystal edges and surface defects, the reaction boundary moving inwards to the centre of the crystal. This observation is consistent with a homogeneous mechanism of decomposition.