A new model has been developed for predicting the viscosity of individual coal ash particles in the range of 10(4)-10(8) Pa.s. Previous experimental work has demonstrated that there is a critical viscosity for adhesion of ash particles. At temperatures and velocities typically found in coal-fired utility boilers, this critical viscosity has been shown to lie between 10(5) and 10(7) Pa.s. Previous models of coal slag viscosity were derived from data at much lower viscosities (less than 10(2) Pa.s) and, while such models accurately predict viscosity of coal slag at high temperatures, they do not perform well in predicting the viscosity associated with particle adhesion to heat-transfer surfaces in utility boilers. Using viscosity data taken from the literature, a new model has been formulated. The data sets are described in detail as well as the coefficients needed to calculate viscosity. Finally, the results of the model are compared to results from previous coal slag viscosity models. The new model is shown to produce a better fit between calculated and measured values of viscosity in the range of 10(4)-10(8) Pa.s. Furthermore, the new model does a better job of predicting viscosity for certain compositions which are commonly found in ash particles.