Materials that show oxide-ion mobility (solid electrolytes) and/or reversible uptake and release of oxygen (oxygen storage materials) have important uses in applications relating to energy and the environment, such as in solid-oxide fuel cells and catalysis, such as automotive exhausts, oxidation of organics and purification of hydrogen. Our preliminary experiments on some new materials reveal that loss and uptake of oxygen occurs very easily at moderate temperature and that the crystal structure of the materials change. Although we have crystal structure models for the materials, because they contain heavy metal atoms the location of oxygen atoms is presently poorly defined. Hence neutron diffraction is needed to resolve the structures fully and understand the origin of the oxygen storage property: we will study the materials in situ in gas flow to follow the oxygen loss in real time.