Silver-based Zeolites are used in filtered containment venting systems to retain fission products. The high silver content of the filters can also facilitate the hydrogen and carbon monoxide catalytic oxidation reactions, hence reducing the explosion risk in the containment and in the filtered vents. The removal of hydrogen and carbon monoxide was studied in two zeolites, AgX (39%wt. Ag) and AgR (11%wt. Ag), under conditions relevant to severe accidents. Experiments were performed at pressures of 1.1 and 3 bar in a fixed-bed tubular catalytic reactor at a constant residence time of 100 ms and a gas preheat of 130 °C. The reactive mixtures comprised H₂/air/steam, H₂/CO/air/steam and CO/air. For tests at 1.1 bar and for the AgX zeolites, it was found that: a) a steady reacting state could be reached for the H₂ reactive mixtures and for the H₂/CO blends with CO volumetric contents less than or equal to 2%, b) for higher CO contents (4 and 5 vol.%) in the H₂/CO blends, exothermic reactions were initiated but they slowly deactivated (at a timescale of 0.5 to 2 hrs), and c) for mixtures with only CO, vigorous combustion was initiated but then the system deactivated in about 0.5 hr. For the 3 bar tests and for the AgX zeolites: a) for only H₂ fuel, a high initial reactivity was observed but then reactions were slowly deactivated, and b) the H₂/CO blends remained always inactive. Finally, AgR zeolites showed no detectable recombination of H₂ and CO.