The structure dependence of BaCe0.8Y0.2O3-d (BCY-20), a leading proton conducting material, will be studied as a function of temperature and, critically, controlled atmosphere for the first time. Previous neutron studies and our own preliminary PXRD indicate that the RT structure is influenced by the level of hydration. We now wish to follow the structural evolution under realistic operating conditions using the recently developed in-situ gas flow cell and deuteration capabilities and request five days on HRPD. The resolution of HRPD is required to follow the complex sequence of phase transitions and differentiate between the chemical expansion, linked to protonation and reduction of Ce4+, and thermal expansion effects.