To design a recombiner packed with hydrophobic catalyst for passive tritium oxidation, the development of manufacturing technology for hydrophobic platinum catalyst and the following analysis on reaction rate are essential. This presentation deals with the reaction rate for tritium oxidation over a hydrophobic platinum catalyst at temperature between room temperature and 473K. Two kinds of hydrophobic platinum catalysts; supported with styrene-divinylbenzene and with silica were used for this test. The results indicate that 1) the particle size of platinum affects the reaction rate, 2) the order of reaction is 0.5 of hydrogen concentration at room temperature in case of a hydrogen content below 100ppm. The rate-determining step approximation method is applied for the discussion on the reaction mechanism of tritium oxidation over platinum catalyst.