Cerium-based oxides are important components of many catalysts they can supply or remove oxygen during redox reactions. During catalysis, oxygen vacancies are constantly created and then refilled in the oxide nanoparticles and the activity of the catalyst is directly related to the ease with which these vacancies can be generated. The high vacancy concentration may lead to substantial phase transformations in the active form of the material. We are engaged in studying the activity of cerium-based oxides with potential applications to solid oxide fuel cell anodes. Using environmental transmission electron microscopy we are able to determine the activity on individual nanoparticles and determine the structure and composition of active phases. In parallel with our experimental work, we are using density functional theory to determine the effect of different dopants on the oxygen vacancy formation energy at both the bulk, surface and interface regions.