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The influence of hydrogen over platinum and combined platinum-palladium diesel oxidation catalysts were investigated on the oxidation kinetics of CO, HC and NO. Although H 2 has been reported to have a positive effect on CO and HC oxidation as well as NO 2 formation over platinum catalysts, there is still uncertainty whether this is due to the temperature rise caused by H 2 oxidation or the result of a change in the reaction kinetics of CO, HC and NO oxidation by the production of intermediate species. The results have showed smaller H 2 concentrations are more effective in improving the catalyst light-off temperature as well as promoting NO oxidation over both platinum and platinum-palladium catalysts. It is suggested that these benefits are a result of not only the exothermic reactions which in turn increase the local catalyst temperature but also H 2 increasing the rate of reactions and the species accessibility to the catalyst active sites thus further CO, HC and NO oxidation can occur at lower catalyst temperatures.