The mobilisation of sodium and potassium during coal combustion and gasification has been modelled using the FACT thermodynamic databases and computation package. Account has been taken of the formation of silicate and fused salt melts and consideration given to deposition taking place during the subsequent cooling of the gases formed. Under combustion conditions, sodium and potassium are predicted to be mobilised to a small extent. The mobilised fraction is predicted to begin to be deposited as salt melt from about 1100 K. The mobility agrees with the limited amount of published data. Under gasifier conditions the predicted mobilisation is greater than observed for potassium-possibly due to the slowness of its release from clays.