In the case of a fire accident in a fusion plant, tritiated organic substances will be produced. We have developed a Pd/ZrO2 catalyst applicable for the oxidation of tritiated organic substances. In this study, two different weight ratios of palladium, 5 and 10 g/l, were selected. The overall reaction rate constant of tritiated methane oxidation with the palladium catalysts in a flow-through system were determined as a function of space velocity from 1200 to 7000 h-1 , methane concentration in carrier from 0.004 to 100 ppm, and temperature of catalyst from 323 to 673 K. As-received catalysts showed a large overall reaction rate constant over the whole tested temperature range. However, the constants gradually decreased after a while. The considerable decrease was evaluated especially over the lower temperature range. The decrease has been explained as caused by the layers of produced water that formed on the surface of the catalyst playing the role of obstacle to reactant transport onto the noble metal deposited on the catalyst. The performance of 10 g/l catalyst was superior to that of 5 g/l over the whole tested temperature range. The overall reaction rate constant was dependent on the space velocity and independent of methane concentration in the carrier.