A rotating cylinder apparatus has been designed and constructed for measuring the viscosity of molten slags under steam pressure in the range 15 to 300 psi and up to about 1400/sup 0/C. Two synthetic coal slags, one high in alkali and the other lower in alkali but higher in calcia were studied with this apparatus. In the high alkali slag equilibrium was probably not achieved and the viscosity appeared to increase with time. In the low alkali slag although the viscosity appeared to be increased with increasing steam pressure the ambient and higher pressure curves appear to tend to coalesce with increasing temperature.