It has been shown previously that metastable rs-Fe 1-x Zn x O (x = 0.6, 0.5, 0.4, 0.3) are kinetically stable up to temperatures of about 700 K . At higher temperatures they decompose into an equilibrium mixture of two solid solutions: cubic (depleted ZnO) and wurtzite (enriched ZnO). In the case of solid solutions with low (x = 0.2, 0.1) ZnO content, a more complicated behavior is observed . Due to the thermodynamic instability of cubic "FeO" (wüstite) at temperatures below 843 K, weak lines of α-Fe and spinel (Fd-3m) phase (probably of Zn 1-y Fe y Fe 2 O 4 composition) appear in the X-ray diffraction patterns when these solid solutions are heated above 700 K. Then the intensity of these lines decreases and at temperatures above 900 K they disappear completely . At the same time, the formation of solid solutions with wurtzite structure is not observed up to 1100 K (the maximum temperature reached in the experiment).