Two Ru/CeO2 catalysts with Ru loadings of 1.65 and 5 wt% were prepared by wet impregnation. The adsorption of propylene or toluene led to two different ESR signals corresponding to two different catalytic sites. These two sites are present among the well dispersed ruthenium oxide species that reduce at similar to 74 degrees C. One site is un-reactive unless activated to remove adsorbed water while the second site is reactive at room temperature. Both sites are easily regenerated under oxidative atmosphere at 200 degrees C. These two sites are optimally obtained for calcination temperatures in the 500-600 degrees C interval and a Ru loading of 1.65 wt%. For Ru5Ce, the same reactive sites are present along with agglomerated ruthenium oxide species that are less reactive in oxidation reactions.