Measurement of the combustion performance of biofuel oils, blends with diesel fuel and emulsions with water have been made, using a variety of experimental techniques. Photographic examination of single droplets demonstrated similar burning rates to diesel fuel. High speed records revealed the explosive combustion of oil-water emulsion droplets. Spray-flame photography showed up the poor combustion efficiency at atmospheric pressure, of the seed oils. compared with diesel fuel. Combustion of sprays at elevated pressure, in a steady-flow combustor, of fuels, blends and emulsions showed improved combustion under these conditions though still poorer for vegetable oil than for diesel fuel. Soot formation levels were also higher, though these were reduced by emulsification with water. Under the more extreme conditions of the environment in a diesel engine combustion chamber, the performance of a commercially available biofuel matched more closely that of diesel fuel. In single-cylinder engine tests at relatively low power and speed, the ignition delay was longer for the biofuel. The difference became less at higher loads and speeds and also in a production line multi-cylinder engine under normal operating conditions. Soot levels were generally higher than for diesel fuel and oxides of nitrogen lower. Unlike most operating modifications which result in a trade-off between these species, emulsification of the biofuel with water reduced levels of both soot and oxides of nitrogen.