A mass-spectrometric investigation of phase equilibria in (barium oxide + diyttrium trioxide) is described. Standard molar enthalpies and standard molar Gibbs free energies of formation were determined for Ba3Y4O9(cr) and BaY2O4(cr). A (p, T, x) phase diagram for (barium oxide + diyttrium trioxide) was constructed in p(T) and T(x) projections. A quasi-three-dimensional representation of the (p, T, x) diagram by means of (T, x)(p) sections (Section-Stand Method) was proposed.