At ambient pressure, the recovered metastable rs-ZnO (sample # 1) remains kinetically stable for at least six months (no further time evolution was studied), and upon heating undergoes a transition to w-ZnO only above 800 K, while nanocrystalline rs-ZnO previously synthesized from w-ZnO nanoparticles is kinetically stable only up to 360 K. Such high thermal stability of rs-ZnO allows a refinement of its p-V-T equation of state (Fig. ). The lattice parameter at ambient conditions agrees well with the extrapolation of previously reported high-pressure -high-temperature data, and is most probably, related to a well-crystallized bulk rs-ZnO, in contrast to previous observations of a wide range of lattice parameters for nanocrystalline rs-ZnO depending on the synthesis method. The analytical expression for the p-V-T equation of state of the form