In undoped crystals of MgO, x irradiation converts a large fraction of Cr3+in octahedral symmetry to Cr2+. Heating such crystals until a blue thermoluminescence has been observed causes further reduction. Usually 95% or more of the Cr3+is converted by the combined treatments. However, in Cr‐doped MgO one is able to reduce only a small fraction of the octahedral chromium to Cr2+. The Cr2+is reconverted to Cr3+by heating. ESR measurements show that in the same temperature interval nearly all of the Fe1+formed on x irradiation has disappeared. The source of electrons acquired by Cr3+thus appears to be the Fe1+ions. The thermoluminescence is ascribed to the Cr3+‐electron interaction, as suggested by Hansler and Segelken.