The known thermodynamic properties of the system Al₂O₃-SiO₂ were corrected using high-temperature differential mass-spectrometry at the temperature 1800-2200 K and at condensed phase composition varied in a wide range. The obtained experimental SiO₂ activity values agree with the calculated by authors using the polymeric model on the basis of Temkin and Lumsden approaches and using the phase diagram for Al₂O₃-SiO₂ system taken from the literature.