Post cylinder oxidation of unburned hydrocarbons (UHC) was studied using a 35 kW lest 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 (NOxlevel) of the exhaust were modified independent of engine settings. The experiments studied spanned a wide range of moderately lean-bum conditions. The composition at the exhaust port was as follows: O25-9% (engine excess air ratios of 1.28-1.75), UHC 1000-7000 ppm, CO 550 ppm, NOx0-1400 ppm. The temperature varied from 600 to 700°C, In addition, experiments with injection of hydrogen peroxide to promote UHC oxidation in the exhaust system were conducted.