In situ studies on reduction processes in praseodymium-doped ceria and nickel-loaded doped ceria nanoparticles have been investigated using environmental transmission electron microscopy. The presence of pit-like defects on the oxide nanoparticles resulted in amorphization during reduction. Hydrogen-spillover in nickel-loaded praseodymium-doped ceria (PDC) was directly observed. Localized reduction of ceria in a H-2 atmosphere was observed and attributed to gas-nickel-ceria interactions at the three-phase boundary. The spatial extent of the localized reduction zones was experimentally determined. The spillover mechanism is discussed in terms of surface diffusion properties of atomic hydrogen and oxygen species. (C) 2011 Elsevier B. V. All rights reserved.