Post cylinder oxidation of unburned hydrocarbons (UHC) was studied using a 35 kW test engine. The engine was equipped with an insulated exhaust reactor to extend the residence time. The exhaust reactor performance has been characterised under the basic engine operating conditions, and under conditions where temperature and composition (NO, level) of the exhaust were modified independent of engine settings. The experiments studied spanned a wide range of moderately lean-burn conditions. The composition at the exhaust port was as follows: O-2 5-9% (engine excess air ratios of 1.281.75), UHC 1000-7000 ppm, CO 550 ppm, NO(x)0-1400 ppm. The temperature varied from 600 to 700 degrees C. In addition, experiments with injection of hydrogen peroxide to promote UHC oxidation in the exhaust system were conducted. The amount of UHC oxidised in the exhaust system depended closely on the exhaust temperature, residence time and concentration of nitrogen oxides in the exhaust gas. The increased reaction time in the exhaust reactor caused an oxidation of the exhaust hydrocarbons of up to 90% with CO as the major oxidation product, but the reactor efficiency decreased as the engine was operated at leaner conditions, due to lower temperatures and lower NOx levels. Levels of nitrogen oxides above 300 ppm were shown to strongly promote the extent of UHC oxidation. Injection of hydrogen peroxide increased the degree of exhaust reactor oxidation and extended the oxidation further into the lean operation range. A field engine was tested at increased NOx levels, obtained by addition of ammonia to the air intake. No physical modifications of the engine or the settings were performed. The ammonia addition resulted in a reduction of unburned hydrocarbons by approximately 30% with CO2 as the major product. Due to the comparatively low temperature and short residence time of the field engine exhaust system, as well as the absence of CO production, the UHC reduction was attributed to processes occurring in the cylinder or exhaust port, rather than in the exhaust system. Similar levels of UHC reduction prior to the exhaust reactor were observed in the test engine.