The Texaco Gasification Process employs a high temperature and pressure slagging gasifier, in which the viscosity of the slag plays a key role in determining operating conditions. Empirical models are available in the literature and predict slag viscosity as a function of temperature and composition, However, these models are applicable only to homogeneous slags. The formation of crystalline phases during cooling can significantly change the viscosity, and the models often fail to predict correct slag viscosity behavior. The formation of crystalline phases and its effect on slag viscosity were studied in four coal slags. The four coals include SUFCo, Pittsburgh No. 8, and two Powell Mountain coals (A and B). Slag viscosity was measured at temperatures in the range of 1150-1500 degrees C under reducing atmosphere. The slag viscosities of Sufco and Powell Mountain A (PMA) coals exhibited the behavior of a glassy slag, of which the viscosity gradually increases as the temperature decreases. The other two, Pittsburgh No. 8 and Powell Mountain B (PMB), showed the behavior of a crystalline slag with T-cv of 1300 degrees C and 1430 degrees C, respectively. Crystalline phase analysis of the slag samples revealed that extensive network formation of dendrite spinels in Pittsburgh No. 8 and PMB caused a rapid increase in the viscosity, while anorthites in SUFCo and large spinels and elongated particles of corundum in PMA did not affect the slag viscosity as much.