In this study, the application of exhaust gas-assisted fuel reforming in compression ignition engines (CI) has been investigated. Experiments were conducted in a single-cylinder direct-injection (DI) diesel engine fueled by conventional diesel and also by a biodiesel mixture. First, the effects of exhaust gas recirculation (EGR) and addition of small amounts of hydrogen on the combustion and exhaust emissions were explored. With the addition of hydrogen, the now of the main fuel (diesel or biodiesel) was reduced to maintain constant indicated mean effective pressure (IMEP). Thus, in effect the tests involved fuel replacement by hydrogen rather than hydrogen addition. Second, the feasibility of producing hydrogen "on-board" by catalytic exhaust gas fuel reforming was examined by incorporating a laboratory reforming mini reactor in the engine exhaust system. Prototype catalysts and different reaction conditions were examined. The results from the first part of the study showed that partial replacement of the hydrocarbon fuel by hydrogen combined with EGR resulted in simultaneous reductions of smoke and nitrogen oxides emissions (NO,) without significant changes to engine efficiency. In the second part of the study, it was shown that the amount of hydrogen required to achieve these beneficial effects potentially can be produced by exhaust gas-assisted reforming of the hydrocarbon fuel.