In situ X-ray diffraction coupled with Rietveld refinement has been used to study CO₂ capture by CaO, Ca(OH)₂ and partially hydrated CaO, as a function of temperature. Phase quantification by Rietveld refinement was performed to monitor the conversion to CaCO₃ and the results were compared to those derived using thermogravimetric analysis (TGA). It was found that Ca(OH)₂ converted directly to 100% CaCO₃ without the formation of a CaO intermediate, at ca. 600 °C. Both pure CaO and partially hydrated CaO (33.6 wt% Ca(OH)₂) reached the same capture capacity, containing approximately 65 wt% CaCO₃ at 800 °C. It was possible to provide direct evidence of the capture mechanism. The stresses in the Ca(OH)₂ phase of the partially hydrated CaO were found to be more than 20 times higher than its strength, leading to disintegration and the generation of nano-sized crystallites. The crystallite size determined using diffraction (75 × 16 nm) was in good agreement with the average crystallite size observed using TEM (of 83 × 16 nm). Electron diffraction patterns confirmed coexistence of CaO and Ca(OH)₂. The analysis provides an explanation of the enhanced capture/disintegration observed in CaO in the presence of steam.