The operation of diesel engines on gaseous fuels commonly known as dual fuel engines in which diesel fuel was used as the pilot fuel and the gaseous fuel (methane and sometime propane in the present work) was used as the main fuel. The gaseous fuel was inducted in the intake manifold to mix with the intake air. The investigation was conducted on a high speed indirect injection (Ricardo-E6) dual fuel engine and was concerned with the effects of exhaust gas recirculation (EGR) on dual fuel engine combustion and emissions,in particular, the effects of intake air temperature and diluent admissions (N2 & CO2) on combustion and emissions. The use of diluents to displace oxygen (O2) in the intake air resulted in reduction in the O2 supplied to the engine, increased inlet charge thermal capacity (thermal effect), and, potentially, participation of CO2 and N2 in the combustion process (chemical effect). In a separate series of tests the temperature of the engine inlet charge was raised gradually in order to simulate the effect of mixing hot EGR with engine inlet gaseous fuel-air mixture. It was found that admission of diluents resulted in reductions in exhaust oxides of nitrogen (NOx). Higher inlet charge temperature increases exhaust NOx but reduces unburned hydrocarbon emissions. Finally, when carbon dioxide was added to the inlet gaseous fuel air charge, large reductions in NOx was observed.