The alumina solubility in the KF-NaF-AlF3 system depending on alkali metal fluoride concentration in the temperature range from 800°C down to melting point of the mixture is presented. The cryolite ratio ([KF]+[NaF])/[AlF3] was kept 1.3 and 1.5. The substitution of cation K+ on Na+ in the KF-NaF-AlF3 melt leads to alumina solubility decrease. The regression equitation determined on the base of literature and obtained data allows estimating the alumina solubility in KF-NaF-AlF3 molten mixtures in the wide CR and temperature interval.