A center has been observed in MgO which has a [100] symmetry and gives rise to mechanical relaxation with associated activation energy of 0.64 eV. The pre‐exponential frequency factor is 1.8×1011sec−1, an anomalously low value which, it is suggested, indicates that the defect reorientation involves a primarily electronic transition. The center is stable only at high temperatures, decaying to a lower thermal equilibrium value below about 850°C. No spin resonance, dielectric loss, or optical absorption associated with the center has been observed, and studies of the effects of various treatments have not allowed the defect to be identified.