Accurate measurement of the viscosity of fuels is essential for the proper operation of fuel supply systems and atomisers. In previous years, the use of fuels derived from the fast pyrolysis of biomass (bio-oils) has been demonstrated in, for example, furnace applications. Unfortunately, these fuels are shown to be unstable and start to polymerise even at ambient conditions. At elevated temperatures, these degradation phenomena become even more significant and complex. For example, vapours will be formed, not only due to the presence of volatiles and water in the oil, but also because of the decomposition of the oil. These vapours make the viscosity measurement. In the present paper, a procedure is proposed for the determination of the viscosity of such (unstable) bio-oils. First, by varying the sample temperature, the so-called decomposition temperature, at which the formation of vapours starts, should be determined. Below that decomposition temperature, the standard procedure for viscosity measurement can be applied. At temperatures above this temperature, the appearance of vapours results in a significant increase and strong fluctuations in the falling time of the ball due to the formation of bubbles. At these latter temperatures, values for the viscosity should be calculated from the lowest value of the falling time during the so-called heat-up period and thr vapour formation delay. The values for the viscosity for five types of bio-oils, derived in this way, were shown to be reliable. (C) 2000 Elsevier Science Ltd. All rights reserved.