In order to understand the results on the unexpected effect of BN on the recovery of rs-ZnO, we carried out a study on the chemical interaction between ZnO and BN under experimental conditions. In general, two possible interactions should be considered: mutual solubility and chemical reaction. The formation of solid solutions is solid solutions between cBN and hypothetical sphalerite (F-43m) ZnO is unlikely due to the large lattice size mismatch (a = 3.615 Å for cBN, and a = 4.563 Å for isostructural ZnO with ionic radii r(Zn 2+ ) = 0.60 Å; r(O 2-) = 1.38 Å in tetrahedral coordination). The chemical reaction could lead to the formation of metallic Zn and boron oxide(s), similar to the interaction between elemental boron and ZnO (3.5 GPa and 1475 K). Chemical interaction between ZnO and BN (both in mixtures and at the capsule/sample contact surface) has been studied over a wide p-T range (4.2-7.7 GPa and 1500-2000 K) in recovery experiments in toroid-type apparatus. No solid-state interaction was observed on the timescale of a few hours. These results indicate that the stabilization of nanostructured rock-salt ZnO is not due to BN-ZnO interaction, which could influence the thermodynamics and kinetics (nucleation/growth) of the reverse transition to w-ZnO; but is mainly caused by mechanical aspects of compression/decompression and sample environment. Also, we should not neglect the time-scale impact and the extreme sensitivity of phase transition s in ZnO to the p-T-time history of the w-ZnO precursor, especially due to thermally induced deactivation of nucleation sites, already at temperatures of ~400 K, and probably lower. In fact, the time scales of multianvil and toroid experiments are very different: 24 h (mainly due to slow compression/decompression) and 1-2 h (fast compression/decompression), respectively. The impact of the p-time history on the kinetics of phase transitions in ZnO has not been studied so far, but is likely to be very important. The faster compression/decompression introduces more defects into the material and thus more nuclei for the phase transition. However, all these observations cannot be explained exclusively by the time scale phenomenon, as shown by parallel experiments in MA and toroid-type apparatuses; the compression conditions (deviatoric stresses and strains, etc.) are also important and influence the nucleation/growth processes on the hour scale. Theoretical simulations and experimental observations show that the uniaxial strain evolution may be intrinsic for direct w-ZnO  rs-ZnO and reverse rs-ZnO  w-ZnO transitions; thus, cold uniaxial decompression may additionally favor the w-ZnO nucleation and reverse transition in toroid experiments.