Nanocrystalline (4-5 nm) Ce1-xRu5O2 mixed oxides (x=0.03-0.16) were synthesized using water in oil microemulsion method. Morphology, microstructure and a phase composition of the samples subjected to heat treatment in oxidizing and reducing atmosphere were investigated by TEM, SEM-EDS-EBSD, XRD and XPS. Oxide with x= 0.11 was structurally stable in oxidizing atmosphere up to 550 degrees C but above this temperature it decomposed into Ru deficient, nanosized Ce1-xRuxO2 and large (few um) RuO2 crystals. No phase separation was observed for Ce0.7Ru0.03O2 even after heating at 800 degrees C. Doping with Ru decreases the size of ceria particles and strongly hinders their sintering at high temperatures. In hydrogen atmosphere a segregation of small (similar to 1 nm) Ru crystallites occurred at the surface of the Ce0.89Ru0.11O2 mixed oxide. Only small increase of the mean crystallite size of Ru (to 2 nm) occurred after reduction at 1000 degrees C. The unique resistance of Ru to sintering is assigned to a special epitaxial orientation Ru (0 0 2)parallel to CeO2 (1 1 1), which persisted up to the highest temperature of reduction, due to very strong surface bonding. Contrary to a situation in oxidizing atmosphere, doping with Ru had no significant effect on the sintering of ceria in hydrogen. Partial substitution of Ru for Ce strongly enhances the reducibility of ceria at low temperatures and its activity in catalytic combustion of CO and soot. (C) 2013 Elsevier B.V. All rights reserved.