A series of engine dynamometer tests was carried out with 100% ethyl ester of soya oil as fuel and six different diesel engine lubricants. In each case the lubricant became contaminated by unburnt fuel during the tests with measured dilution rates of up to 0.2% of the fuel throughput. The lubricant/fuel mixture eventually underwent degradation to such an extent that phase separation occurred. The tests were terminated when the lubricant lost all dispersancy, as evaluated by a blotter-spot teat. Used oil analysis revealed that rapid oxidation of some of the fatty acid ester components of the fuel diluent had occurred in the later stages of the tests. At the high levels of fuel dilution recorded in these tests there was little difference between the performances of the six lubricants, despite their differing performance categories. It is therefore concluded that conventional performance categories cannot be used to define the grade of lubricant necessary for use with 100% vegetable oil ester fuels.