Recent interest in the suitability of Australian bituminous coals for use in integrated gasification-combined cycle (IGCC) technologies has provided the opportunity to determine viscosity data for a range of coal ashes slags fluxed with limestone at the tapping temperatures of entrained flow gasifiers. Experimental viscosity measurements have been made over a range of slag compositions covering the anorthite region at the 0-2.5, 2.5-5, 5-7.5 and 7.5-10 wt% FeO levels of the quaternary SiO2-Al2O3-CaO-FeO system. Contour plots of viscosities at 1450 degrees C for the four FeO ranges are presented as an example for predicting slag behaviour in entrained flow gasifiers. The viscosity measurements have also been fitted empirically using a modified Urbain treatment to give separate models for the four FeO levels. Polynomial expressions are given for the evaluation of viscosities covering the temperature range 1400-1550 degrees C for slags within the compositional range used in the derivation.