Abstract: The evaporation of Y2O3 in the presence of carbon was studied by high-temperature differential mass spectrometry. In the temperature range 1950–2200 K, the vapor above the Y2O3–C system contains YO and atomic yttrium. At elevated temperatures, the partial pressure of YO vapor over the Y2O3–C system decreases compared to the partial pressure of YO vapor over individual Y2O3, leading to a decrease in the activity of yttrium oxide in the system.