Biodiesel is considered as an alternative fuel to Diesel fuel No. 2. which can be generally produced from different kinds of vegetable oils. Since the prices of edible vegetable oils are higher than that of Diesel fuel No. 2, waste vegetable oils and non-edible crude vegetable oils are preferred as potential low priced bio-diesel sources. In addition, it is possible to use soapstock.. a by-product of edible oil production, for cheap biodiesel production. In this study, a methyl ester biodiesel was produced from a hazelnut soapstock/waste sunflower oil mixture using methanol, sulphuric acid and sodium hydroxide in a two stage process. The effects of the methyl ester addition to Diesel No. 2 on the performance and emissions of a four cycle. four cylinder, turbocharged indirect injection (IDI) Diesel engine were examined at both full and partial loads. Experimental results showed that the hazelnut soapstock/waste sunflower oil methyl ester can be partially substituted for the Diesel fuel at most operating conditions in terms of the performance parameters and emissions without any engine modification and preheating, of the blends. (C) 2004 Elsevier Ltd. All rights reserved.