Petroleum coke may be used as a fuel for entrained-flow slagging gasification. It may be blended with coal to provide a more attractive feedstock. The coal provides the benefits of enhancing reactivity and increasing the amount of slag coating the gasifier walls, while the petroleum coke increases the heating value of the fuel blend. The slagging behaviour of the petroleum coke or blend must be known to determine if it is a suitable feedstock. In the present study, the slag viscosities of coal, petroleum coke and coal/petroleum coke blends were measured in the temperature range of 1175-1650 degrees C. Two different viscosity measurement apparatuses were used in separate laboratories. Some viscosity measurements were repeated to test reproducibility of the results. Also, slags with and without sulphur were tested to determine whether the effect of sulphur can be neglected. The largest deviation between repeated log (viscosity in Pa s) measurements is 0.11. Both petroleum coke slags tested are suitable for operation at or above 1275 degrees C. The temperature range of operability for one coal slag tested increased when blended with petroleum coke ash, but did not significantly change for other coal slags tested. Some effects of blending on viscosity behaviour can be explained by network former and modifier theory. Other effects are attributed to solids formation which was investigated via slag quenching experiments and FactSage predictions. Below 1300 degrees C, vanadium, a major component of petroleum coke ashes, promotes the formation of spinel which increases slag viscosity. (C) 2012 Elsevier Ltd. All rights reserved.