Plant oils are considered viable replacement fuels for diesel engines. However, in order to become successful diesel fuel substitutes, problems associated with the formation of lacquer and carbon deposits on engine components must be resolved, else truly long-term engine reliability will not be possible.This paper reports some basic experiments into the formation of residues due to liquid phase reactions of a number of plant oils as a function of temperature.Heating tests on suspended drops of sunflower, corn, olive, and safflower oils were performed. Residue deposits were measured. For a heating air temperature of approximately 300°C, roughly 50% of the original oil drop mass remained as residue. This amount rapidly decreased as the air temperature was increased. Above approximately 500°C small amounts of residue formed which burned off shortly after formation. A methyl ester of sunflower oil also tested formed substantially less residue than any of the neat plant oils.The test results are discussed with regard to possible implications for diesel engines operating on such oils.