This paper reports on the results of systematic investigations into the thermodynamic properties of melts lying along the joins with constant SiO ₂ contents of 0.5 and 0.6 mole fractions in the CaO-Al ₂O₃-SiO₂ system at mole fraction ratios CaO:Al₂O₃ varying from 0.25 to 4.0. The activities of SiO₂ in melts of this system at a temperature of 1905 ± 10 K are determined by high-temperature mass spectrometry. It is found that, in the concentration range studied, the activity of SiO₂ remains constant at SiO₂ contents of 0.5 and 0.6 mole fractions.