The Ti-doped HEO2-n sample shows significant chemical expansion of the c parameter up to 600 °C during heating and cooling in O2, as expected from the TGA results (Figure ). Above 600 °C, the increase in the c parameter becomes less distinct, but the continuous increase still indicates that the material has not reached a plateau at its assumed minimum oxygen content of δ = 0.075, as stoichiometric hexagonal manganites show a negative thermal expansion coefficient along the c-axis. The large gap between the c parameter values in O2 and N2 at higher temperatures also indicate that δ > 0 at these temperatures. No large expansion of the abplane is visible during oxidation of this sample, instead a small contraction in the a parameter at T < 300 °C can be observed, similar to what has been reported for Ti-doped DyMnO3. A small contraction of the c parameter can be seen in N2 atmosphere, indicating that some incorporation of interstitial oxygen also occurs at low pO2 (nominally 5.0 purity, ~10 -4 -10 -6 atm). No other changes in lattice parameters occur during heating and cooling in N2 atmosphere, except what can be attributed to thermal expansion. We now investigate the kinetics of oxidation by synchrotron XRD upon in situ switching from N2 to O2 atmosphere. In situ XANES upon atmosphere switch shows progressive oxidation of Mn 3+ to Mn 4+ with time (Supp. Fig. ). Diffractograms for HEO1-n and HEO2-n in collected at 200 °C and 300 °C are shown in Figure , more temperatures are presented in Figures S6-S9 in the Supporting Information. Both samples show shifts in peak positions upon O2 exposure, a signature of interstitial oxygen entering the lattice. The oxidation time decreases with increasing temperature for both samples, and the Ti-doped sample oxidizes faster than the undoped sample, as expected. The visible R3c reflections at ~6.8° 2θ and ~6.9° 2θ in N2 atmosphere for HEO1-n indicate that the material contains significant amounts of interstitial oxygen even before being reexposed to oxygen, indicating that both the initial rate of oxidation and the total time for oxidation are underestimated.