The viscosities of nine coal ash slags were measured in air as a function of temperature. Selected slags were also tested in reducing atmosphere and in air with 10% water vapor. In air, the viscosities of the slags can be related to the mole ratio of basic to acidic oxides. An improved fit is obtained from the ratio of oxide ions contributing to the bases to the oxide contributing to the acids. The change in viscosity observed in using hydrogen instead of air appears to be related to the amount of iron originally in the slag. When water vapor is present in air, the viscosity of a high-silica glassy slag decreased. The viscosities of three other slags, with lower silica content, increased slightly, with the increase in potassium content. In the former case, water likely enters the slag and disrupts Si-O-Si bridges. In the latter case, water may form metal-hydroxide complexes with the alkali cations such as potassium. A second effect of water vapor is to reduce the temperature at which the slag begins to crystallize (the temperature of critical viscosity). These effects of water on coal ash slag viscosity are similar to those on magmas, even if with lower magnitude.