Results are presented of tests on a single-cylinder direct-injection diesel engine operating on diesel fuel and also a commercial seed-based oil in emulsions containing 5, 10 and 15% vol water. The tests covered a range of loads at two speeds. Values are given for ignition delay, specific fuel consumption, thermal efficiency and equivalence ratio, and the concentrations of carbon monoxide, carbon dioxide, oxides of nitrogen and smoke particulates in the exhaust gases. At the lower speed, ignition delay was longer for the vegetable oil and its emulsions. At the higher speed the neat fuels had similar ignition delays, but for the emulsions the delays were longer. Carbon dioxide emissions were similar for all fuels (although vegetable oil, a renewable energy source, may be a lower net contributor to atmospheric levels). Emissions of oxides of nitrogen were fewer and smoke levels were higher for vegetable oil. Emissions of both were lower for the emulsified fuels.