Barium aluminosilicates (BAS) were coated on the carbon fiber-reinforced silicon carbide composites (C/SiC) as environmental barriers. The hot corrosion behavior of the coated composites was studied at 900°C in dry air and water vapor, respectively. The molten Na2SO4 was used as the corrosion reactant. The results indicate that the BAS coatings can effectively block the attack of molten Na2SO4 to C/SiC composites in dry air. However, the coated composites degrade rapidly when exposed to molten Na2SO4 coupled with water vapor. It is found that the BAS is corroded by Na2SO4 melt with the formation of BaSO4, resulting in the destruction of BAS structure, which makes the coating lose its protection to the C/SiC composites in water vapor.