A water-in-fuel emulsion composition containing a Fischer-Tropsch derived fuel and water, and its use in a compression ignition engine is provided. Emissions, for example of NOx, black smoke and/or particulate matter, are lower as compared to conventional fuels but without lengthening the ignition delay and reducing the cetane number. This is achieved without the need for, or at reduced levels of, ignition improving additives, and without engine modifications.