Vegetable oils, in their pure form, are increasingly used as a cost-effective, locally produced, renewable fuel. Early results revealed gradual formation of engine deposits while operated on vegetable oils. While current practices, level of sophistication, and results vary among individual “conversions”, the most sophisticated installations operate in dual-fuel mode, with heated vegetable oil introduced into an already warm engine. Heating of the vegetable oil and pre-heating of the engine on ordinary fuel have resulted in operational and emissions improvements. To evaluate the effects on emissions, tests were conducted on several modern private passenger vehicles with direct-injection turbodiesel engines, which were retrofitted with a secondary heated fueling system, but with no other modifications or adjustments to the engines and injection pumps. While operating on vegetable oil, engines generally exhibited slightly lower NOx and higher CO than while compared to diesel fuel. The effects on the emissions of hydrocarbons (HC) and particulate matter (PM) were inconsistent. Analysis of real-time data suggests that HC and PM emissions increase during low speed, low load operation, and decrease at higher engine speeds and loads. Qualitative assessment of PM composition suggests higher relative fraction of organic, semi-volatile PM, and lower fraction of soot. When of concern, increases in organic PM, HC and CO can be effectively mitigated by oxidation catalysts. During prolonged idling, however, engine-out PM emissions can radically increase, while catalyst efficiency may diminish. It appears that operation on heated vegetable oil at highway speeds results in the reduction of all regulated pollutants and greenhouse gas emissions; operation in congested urban areas deserves further investigation.