Two-colour pyrometry, thermodynamic analysis, and exhaust emissions analysis have been used to improve understanding of the formation of soot during combustion in a high speed direct-injection automotive diesel engine. Three fuel blends were used: a Base Fuel commercially available in Northern Europe; a blend of the Base Fuel (70%) and esterified rape-seed oil (RME) (30%) and a blend of the Base Fuel (90%) and an ether compound (diglyme) (10%). While the Base Fuel contained no oxygen, both the other two fuels contained equal amounts of oxygen of 3% by mass. The principal results show significant differences in soot generation during combustion between the two oxygenated fuel blends, despite both having the same amount of oxygen. (c) 2006 Elsevier Ltd. All rights reserved.