It has been suggested that the alumina added to aluminium electrolysis cells should be pre-heated to obtain more rapid dissolution and thereby smoother operation. To investigate the effect of pre-heating, the formation of alumina "rafts" upon feeding a batch of alumina to the surface of molten cryolitic bath was studied in the laboratory. The alumina batches were heated to pre-determined temperatures, and the rafts formed were collected by means of a plate-shaped sampler. It turned out that coherent rafts were formed up to at least 500 °C. Since alumina dissolution is mass transfer controlled, this means that preheating the alumina will have no effect on its dissolution rate until the temperature is at least 500 °C. Preheating to such temperatures represents a great challenge in industrial cells.