Measurements of the rate of dissolution of single crystals of sapphire in molten CaO‐A12O3‐SiO2slag from 1340° to 1550°C under conditions of molecular diffusion, free convection, and forced convection reveal that the rate of dissolution was controlled by transport in the molten silicate. The results were self‐consistent in that the independently calculated apparent binary diffusion coefficient agreed well for all three cases. This binary diffusion coefficient at temperatures above 1450°C coincided remarkably well with that of the tracer diffusion coefficient for the silicon ion. For a number of reasons the significance of this agreement should not be overe