To realize the objective of high alumina yield and a high molar Na2O/Al2O3 ratio in caustic solutions for Bayer plants, this paper presents the solubility of Al2O3 in methanol-water solvent mixtures with a fixed mass ratio of methanol to water 1:1 at (30 and 60)degrees C and containing various Na2O concentrations. The Al2O3 solubility in the Na2O-Al2O3-H2O-CH3OH quaternary system increases with an increase of Na2O concentration, reaching maximum values of 1.70% and 2.48% by weight at (30 and 60) degrees C, respectively. Then, the solubility decreases with a further increase in the Na2O concentration. The equilibrium solids are aluminum hydroxide Al(OH)(3) and mono-sodium aluminate hydrate Na2O center dot Al2O3 center dot 2.5H(2)O in the different alkali concentration regions. The data show that the solubility of Al2O3 in the Na2O-Al2O3-H2O-CH3OH quaternary system is much lower than that in the Na2O-Al2O3-H2O ternary system.