The effect of cryolite on the formation of aluminium carbide was studied by an aluminium-carbon diffusion couple experiment. The interface between the aluminium and carbon was coated by a thin layer of cryolite, and the diffusion couples experiments were performed at 1030 °C in stagnant argon atmosphere. The microstructure of the relatively thick and porous aluminium carbide formed at the solid-liquid interface was investigated by optical and scanning electron microscopy. It was shown that the presence of cryolite has a pronounced effect on the formation of carbide, although fluoride phases were not found at the interface after the experiment. The porous and needle like morphology of the carbide layer suggests the mass transport during growth is through the gas phase. The stability of cryolite and possible gas species involved in the growth process have been analyzed thermodynamically to identify possible gaseous species present at both global equilibrium and metastable conditions.