[en] The oxide layer formed on AGR reactor steels in a CO₂/CO coolant environment contains two major components, a cubic spinel phase and a rhombohedral phase. In general the rhombohedral phase protects the underlying steel from further attack better than the spinel phase. The rhombohedral phase may be Cr₂O₃ or a solid solution of αFe₂O₃ and Cr₂O₃. The composition of the solid solution can be identified using X-ray diffraction techniques (XRD). Standard solid solutions of the rhombohedral phase have been prepared and characterised by XRD. The generation of calibration curves relating lattice parameter or d-spacing to oxide composition enables the composition of the rhombohedral phase formed on samples of reactor steels to be identified. (author)