Perovskite type catalysts La0.7Sr0.3Cr1-xRuxO3 (0.025 <= x <= 0.100) were synthesized by annealing a mixture of metal oxides and carbonates gradually up to 1000 degrees C in air, and characterized by XRPD, XPS, TPD, SEM-EDS and the van der Pauw method. The CO oxidation activity was investigated in a differential recycle reactor. According to the XRPD results, all samples achieved a perovskite structure, with a small presence of SrCrO4 phase. The XPS results revealed that the surface composition of all samples differed considerably from the stoichiometric value with ail important segregation of strontium and mainly ruthenium with regard to chromium at the surface of the catalysts. The sharp decrease of resistivity with increasing surface concentration of ruthenium and the independence of the resistivity on temperature for the sample with x = 0.100 imply the possible presence of SrRuO3, La-Ru-O and highly dispersed RuO2 (invisible by XRPD), known as good electric conductors, at the surface. The CO oxidation activity increases with increasing the degree of substitution (x). The surface concentrations of ruthenium are almost the same in the samples with x = 0.075 and 0.100. Those samples showed the similar values of resistivity in whole investigated temperature ram,e and very close CO oxidation activity. which indicates that the concentration of Ru4+ in the surface region and its stability are determining factors for the CO oxidation activity. The main results of this study are that ruthenium perovskites have a high thermal stability and CO oxidation activity. (c) 2006 Elsevier B.V. All rights reserved.