In this experiment, argemone oil, toxic and highly viscous oil, was treated in two different processes to use this as an adulterer to petroleum diesel. Firstly oil was passed through several processes to lower it viscosity and to improve its properties. The resultant oil, refined argemone oil (RAO), was compared with traditional transesterified biodiesel (TEO) prepared from crude argemone oil. Results indicated that RAO had properties closer to those of TEO and important properties like viscosity, acid value, iodine value and pour point of RAO were also within the acceptable range. Engine testing revealed that up to 42% increment in engine runtime was recorded when only 10% RAO was added to the petroleum diesel, and this is probably the most feasible technique to convert argemone oil to biofuel additive with petroleum diesel. GC-MS analysis revealed the presence of methyl esters of several saturated and unsaturated fatty acids in both RAO and TEO. However, methyl eicosanoate was present in the highest concentration in these oils. (C) 2011 Elsevier Ltd. All rights reserved.