The activities of silicon dioxide and barium oxide in the BaO-TiO ₂-SiO₂ system containing from 10 to 50 mol % SiO ₂ at a mole fraction ratio BaO : TiO₂ = 1 and at a constant SiO₂ mole fraction of 0.5 are determined using high-temperature differential mass spectrometry. It is shown that the positive deviation of the SiO₂ activity and the negative deviation of the BaO activity from ideal behavior are observed at high SiO₂ contents.