The ZnO-MeO solid solutions synthesized at high pressure and high temperature with subsequent quenching are kinetically stable under ambient conditions over a rather wide temperature range. In general, all metastable phases, including dense high-pressure phases, should undergo phase transformations into thermodynamically stable equilibrium phases. In the case of rs-Me 1-x Zn x O solid solutions, we will deal not only with the rock-salt → wurtzite phase transitions, but also with the decomposition of supersaturated wurtzite solid solutions upon temperature increase. This decomposition can proceed by different mechanisms, including the stabilization of supersaturated solutions. The removal of existing kinetic barriers of phase transition/decomposition can be achieved by heating to high temperatures. The driving force for such processes is the difference in Gibbs free energy (ΔG) between high-pressure and low-pressure phases. The sign and magnitude of ΔG are determined primarily by the magnitude and sign of the difference in molar volumes of these phases.