The aim of the HoTMiX academic research project is to provide a deep understanding of the relationships between the nonlinear mechanical response of oxide materials at very high temperature and their microstructure at the nanoscale. During elaboration, or operando conditions, solid-state phase transitions (SPTs) associated with highly anisotropic elastic behavior and thermal expansion induce a complex mechanical response that still remains to be studied (and eventually tailored). Relaxation of thermal stresses through SPTs results in the formation of microstructures that usually involve length scales spanning at least three orders of magnitude from the crystal to the grain size. Indeed, coherently diffracting domains have a typical size of few tens of nanometers and they are part of larger crystalline areas of usually a few tens of micrometers. A striking feature of these oxide materials lies in the huge (three orders of magnitude) difference between local stresses within coherent domains (nm scale), which are in the GPa range, and the tensile strength of the bulk (i.e. cm scale), usually of only a few tens of MPa. On top of that, the temperature range in which relevant phenomena take place (stress build-up, microcracking, and SPTs) span from room temperature to 2000 °C, thereby covering three orders of magnitude. Therefore, the general question that we want to address requires a multiscale analysis along three main axes: temperature, stress, size. Combining plasticity at the microstructural scale with unconventional elastic behavior, related to size effects, some intrinsically brittle oxide materials exhibit an unexpected high compliance. Although this is observed at the microscale, its origin lies at the nanoscale. The understanding at the nanoscale of this mechanical behavior, is the central objective of the HoTMiX project. Using several X-ray based advanced techniques (scattering, diffraction, refraction, tomography) at synchrotron radiation beamlines, we will determine the microstructure evolution (in situ at very high temperatures and/or under applied stresses) of these “living oxide materials”. This is the main experimental challenges that we will address in the HoTMiX project. The relationship between microstructure and mechanical properties will be explored by combining in situ quantitative experimental measurements at very high temperature and/or under applied stresses with accurate microstructural modelling based on virtual but realistic microstructures submitted to temperature and external stresses evolutions.