A Method was developed for measuring the capillary depression of Met.allic aluminum in an alumina tube in a cryolite melt. As the tube was progressively lowered through the melt into the Met.al the volume of gas expelled was measured by the movement of a meniscus of liquid in a horizontal glass tube. No movement occurred after the tube entered the aluminum until it was far enough down that the Met.al could enter it. A correction was applied for the finite radius of the crucible holding the melt, the necessary theory being derived. The contact angle between Met.al, alumina, and cryolite was determined from the shape of the Met.al frozen in the crucible. It was found that the contact angle of the Metal on alumina is very close to 180 deg, and that the interfacial tension at 1000°C is 460 ± 27 mN · m-1 (standard deviation). Interfacial tension decreases with increasing NaF/AlF3 ratio, and increases with addition of MgF2 or Li3AlF6. CaF2 has no significant effect. The hypothesis is advanced that the effect of ratio is due to adsorption of Na atoms, generated by the reaction 3 NaF + Al ⇌ A1F3 + 3Na, at the interface. Application of the Gibbs adsorption isotherm suggests that at molar ratios NaF/AlF3 above 2.8 the interface is covered with a monolayer of sodium atoms.