- Three ternary intermetallic phases were revealed in Mg-20 at% Al—20 at% Ag alloy: Mg 59 Al 34 Ag 7 (body-centered cubic, a= 10.2 Å, probably isotypical to Mg 17 Al 12 phase); Mg 62 Al 11 Ag 27 (body centered cubic, a= 14.5 Å) and new Mg 19 Al 13 Ag 6 compound. The geometry and the symmetry of the unit cell of these phases were investigated by electron diffraction methods. It was found that the structure of the new Mg 19 Al 13 Ag 6 phase belongs to the orthorhombic crystal system with lattice parameters of a= 8.87, b= 16.48 and c= 19.48 Å. The space group was evaluated by Precession Electron Diffraction technique in “descan-off” mode as Pbcm (No. 57).