We report our results of the synthesis of I : I molar ratio of the silver cobalt and silver manganese composite oxide catalysts to remove carbon monoxide from hydrogen-rich fuels by the catalytic oxidation reaction. Catalysts were synthesized by the co-precipitation method. XRD, BET, TGA, catalytic activity and catalyst deactivation studies were used to identify active catalysts. Both CO oxidation and selective CO oxidation were carried out in a microreactor using a reaction gas mixture of 1 vol% CO in air and another gas mixture was prepared by mixing 1 vol% CO, 2 vol% O-2, 84 vol% H-2, the balance being He. 15 vol% CO2 was added to the reactant gas mixture in order to determine the effect Of CO2, reaction gases were passed through the humidifier to determine the effect of the water vapor on the oxidation reaction. It was demonstrated that metal oxide base was decomposed to the metallic phase and surface areas of the catalysts were decreased when the calcination temperature increased from 200 degreesC to 500 degreesC. Ag/Co composite oxide catalyst calcined at 200 degreesC gave good activity at low temperatures and 90% of CO conversion at 180 degreesC was obtained for the selective CO oxidation reaction. The addition of the impurities (CO2 or H2O) decreased the activity of catalyst for selective CO oxidation in order to get highly rich hydrogen fuels. (C) 2001 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.