Abstract: For decades the materials paradigm has been a linear process used to connect structure to properties and performance. However, this wrongfully assumes that the operating conditions under which the materials work do not affect the structure measured under ambient or mild conditions. In this work, several oxygen transport membranes have been studied under industrially relevant conditions for the partial oxidation of methane via in situ neutron and synchrotron diffraction. Both temperature and gas atmospheres were modulated, and the weight percents of all intermediate phases were determined. These studies have shown that not only are the structures under extreme operating conditions far from the ambient structures, but they have also led to structural mechanisms that properly assign the catalytically active phase of industrially important materials. In addition to mechanistic insight, we have established key differences that exist between in situ diffraction experiments at synchrotron and neutron sources that should be considered when designing experiments.