The increased interest in the Integrated Gasification-Combined Cycle (IGCC) power generation technology has led to a need to predict viscosities for coal ash and fluxed coal ash slags at the tapping temperatures of entrained flow,gasifiers. Slags derived by fluxing Australian bituminous coal ashes with limestone have properties that may be approximated by the SiO2-Al2O3-CaO-FeO system. This note presents experimental determinations of the viscosity and the temperature of critical viscosity for artificial mixtures covering the anorthite region at the 5 and 10 wt.% FeO levels of this quaternary system. The viscosity measurements were fitted empirically using a modified Urbain treatment. Contour plots of viscosities at 1400 degrees C, 1450 degrees C and 1500 degrees C and of temperatures of critical viscosity at the 5 and 10 wt.% FeO levels are presented as an aid for predicting slag behaviour in entrained flow gasifiers. The viscosity results also form a self consistent data base for future theoretical studies.