[en] A vertical section at 33.3 at.% Y in the Au-Ag-Y ternary system was studied using X-ray diffraction analysis, differential thermal analysis, and optical microscopy. An allotropic phase transformation of the Au₂Y phase in the Au-Y binary system is found. The Au₂Y phase transforms from a room temperature phase α to a high-temperature modification β at about 778 deg. C. The α-Au₂Y has a tetragonal MoSi₂-type structure with the lattice parameters a = 0.3692 nm, c = 0.8981 nm; the β-Au₂Y has an orthorhombic CeCu₂-type structure with the lattice parameters a = 0.4658 nm, b = 0.7340 nm, and c = 0.8200 nm. No allotropic phase transformation of the Ag₂Y phase in the Ag-Y binary system is found. The vertical section of the Au₂Y-Ag₂Y in the Au-Ag-Y ternary phase diagram established on the basis of these studies consists of seven phase regions: L (liquid phase region), α (MoSi₂-type structure), β (CeCu₂-type structure), α + β, α + L, β + L, and α + β + L