The investigation was prompted by the absence of correct experimental data on component activities in MgO-SiO₂ system. The experiments were conducted using differential mass spectrometry technique in the temperature range 1700-2200 K. The dependences of MgO and SiO₂ activities and of Gibbs fee energy and excess Gibbs energy on the condensed phase composition are analyzed and it is shown that the obtained thermodynamic characteristics are in agreement with presently adopted state diagram.