The twin challenges of fossil fuel depletion and environmental degradation, present engine and vehicle manufacturers with problems focused on future provision of both automotive power plant and conventional hydrocarbon fuels. In the drive to meet more stringent emission controls, many options have been identified, in the investigation of viable alternative fuels, and in the means of meeting the standards. While the operation of spark-ignition engines with natural gas is proven, other fuels are currently being explored for compression-ignition engines. In this study, di-methyl ether and rape-seed methyl ester, together with diesel fuel, were used, both as neat fuels and for pilot injection, in a natural gas dual-fuelled compression-ignition engine, to examine the performance and the levels of exhaust emissions. The merits and shortcomings as alternative fuels for diesel engines are discussed. While the ester performs much like diesel fuel in both modes, the ether produced lower specific oxides of nitrogen in dual-fuel operation. (C) 2007 Elsevier Ltd. All rights reserved.