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As legislation demands continually lower emissions of regulated pollutants, i.e. hydrocarbons, NOx, CO and particulates, vehicle and engine manufacturers have turned to catalyst technology in an attempt to achieve the ever more stringent limits. This technology has now been extended to diesel powered vehicles and a number of manufacturers are fitting oxidation catalysts as standard. This work presents experimental data to show the effectiveness of oxidation catalyst technology in reducing exhaust HC emissions from a modern high speed diesel engine. It tested the ability of the catalyst to reduce HC emissions at a range of steady state speed and load combinations to assess the controlling factors in catalyst efficiency. The work found that exhaust gas temperature is the most significant controlling factor in determining catalyst efficiency. This is significant because at low speed and load the exhaust of a diesel engine is below the light off temperature of the catalyst.