The rotating ring-disc electrode technique has been used to investigate the reaction mechanism of the aluminium electrodeposition process in cryolite-based electrolytes. Laboratory studies using high temperature gold-molybdenum and platinum-molybdenum rotating ring-disc electrodes have provided evidence for the existence of a subvalent intermediate species (A1(I)). In a cryolite-alumina electrolyte (bath ratio : 1.5), two well separated convective-diffusion controlled oxidation processes were observed at both a gold and a platinum ring during aluminium electrodeposition at the disc. On the basis of the data presented, a reaction scheme involving reduction of A1(III) to A1(0) via A1(I), followed by chemical dissolution of A1(0) into the bulk electrolyte was proposed. The loss of current efficiency in aluminium smelting was primarily attributed to the chemical dissolution of A1(0), rather than to the formation of a subvalent intermediate species.