All investigated nanocrystalline samples absorb and release oxygen fast (Figure ), with no significant loss of OSC when increasing the cooling rate up to 20 °C min -1 . In contrast, literature reports on undoped Y-, Ho-or Dy-based materials all show significant reduction in OSC during cooling even at 5 °C min -1 cooling rate. YMnO3 substituted with larger R such as Tb, Gd and Sm show no significant change in OSC when increasing the cooling rate from 1 to 5 °C min -1 , 28 indicating that the larger R improves the kinetics of oxidation. However, these R-substituted samples also show a significant increase in thermal hysteresis with increasing cooling rates, implying significantly slower oxidation kinetics than for the highentropy materials studied here. Thermal hysteresis upon heating and cooling shows that the oxygen exchange is not fast enough to maintain equilibrium oxygen content and the lack of such hysteresis for the Ti-doped HEO2-n samples (Figure ) implies that Ti 4+ further improves the oxidation kinetics, and that also oxygen release is fast. Substituted large R 3+ cations such as Gd 3+ or Sm 3+ have also shown indications of slower oxygen release due to stabilization of the oxidized phase. Both bulk samples show fast oxygen absorption at 5 °C min -1 cooling rate (Figure ) and only a subtle decrease in oxygen content with faster heating and cooling rates of 10 and 20 °C min -1 . This shows that even bulk powders with micrometer-sized particles display remarkably fast oxygen exchange. Compared to literature reports for 5 °C min -1 cooling rate, oxygen contents of δ = 0.28 for HEO2-b and δ = 0.22 for HEO5-b are comparable or better than many nanocrystalline single R hexagonal manganites, and significantly better than other reported bulk materials. This suggests the use of bulk high entropy hexagonal manganites, prepared by cheap and easily scalable solid state methods, for industrial oxygen separation applications.