Chemical expansion of the c axis (Figure ) can be directly correlated with the degree of oxidation, and the surface exchange rates of oxygen for the two samples can be calculated using a model developed by Moreno et al. Calculated surface exchange rates are shown in Figure and the inferred activation energies of HEO1n and HEO2-n are 0.59 and 0.58 eV, respectively. In comparison, the activation energies of single-R nanocrystalline HoMn0.85Ti0.15O3 and DyMn0.85Ti0.15O3 are 0.54 and 0.68 eV. The surface exchange rates are higher for high entropy compositions than both Ti-doped and undoped single-R counterparts, and comparable to values reported for perovskites and other oxides at several hundred degrees higher temperatures. The surface exchange rates for HEO1-n are in addition likely an underestimation, as they are calculated from only the last, and slower, part of the oxidation process. Oxygen storage capacity. All samples show large OSC, with the highest found for HEO4-n with δ = 0.37 (Figure ). Such high OSCs are commonly found for hexagonal manganites with large R 3+ cations (rR3+ ≥ rDy), and can thus not be attributed to high configurational entropy. Oxidation of Tb 3+ to Tb 4+ may also contribute to high OSC, supported by the XANES spectra in Supp. Fig. ) and the increasing OSC with increasing Tb-content (Figure ). It is not known where in the crystal structure Oi charge compensating Tb 4+ are found, e.g. if Oi resides in the Mn-O layer or in the R-O layer. As Tb is found as both Tb 3+ and Tb 4+ at T > 900 °C in high pO2, the dashed minima lines, corresponding to all Mn being present as Mn 3+ , in Figures 1-3 may underestimate the oxygen content at higher temperatures.