Biomass oil derived from the pyrolysis of sewage sludge has good commercial value as a fuel to power diesel engines. However, the properties of the oil, such as its bad odour, high viscosity and its instability can be a disadvantage for marketing the oil. This paper examines the possibilities of improving the characteristics of biomass oil that was derived from sludge using oil from sludge (OFS) technology. Esterification of the pyrolysis oil with ethanol and sulphuric acid (as a catalyst) was found to improve the odour characteristics significantly. Detailed analysis using gas chromatography-mass spectrometer (GC-MS) suggested conversion of fatty acids in the pyrolysis oil into esters. Among the alcohols tested, ethanol was found to produce the best smelling oil. An attempt has been made to determine the optimum parameters for the esterification process. The optimisation included the quantity of ethanol and catalyst required, in addition to the required time and temperature for the reaction. The method was found to improve the odour of the oil from an extremely annoying level to a not annoying level, reduce the viscosity of the oil by four times and increase the heating value of the oil up to 9%. Another improvement observed was improved stability of the oil, making it more suitable to be stored for long-term use. (c) 2004 Elsevier B.V. All rights reserved.