Sulfur hexafluoride (SF6) is widely used by the magnesium industry as a component of cover gas mixtures, which reacts with the melt and forms a protective film on the melt surface. In this study the dew points of the air that acts as carrier gas for SF6 was varied from 203K to 273K, and the effect of the SF6 cover gas humidity and concentration on the efficient protection of molten AZ91D magnesium alloy was investigated in temperatures ranging from 903K to 973K. Gas chromatography (GC) combined with ion chromatography (IC) were used to identify the chemical constituents of the by-products present in the protective atmosphere, while. X-ray photoelectron spectroscopy (XPS) and Auger electron Spectroscopy (AES) were used in the characterization of the protective films formed on the melt. The results indicate that the quantity of SF6 to protect molten Mg from burning decreased with decrease in the humidity of the cover gas XPS showed that the films formed in all atmospheres with different dew points were composed of MgO, MgF2 and MgSO4. However, in cover gases with higher dew point, the amount of MgO of the film tended to increase, due to the dissociation of MgSO4 of the film into H2 and SO4-2 in the presence of moisture, which lead to the destruction of the protective film and oxidization of the melt.