Results of a study of regular features of formation of phase composition and of evolution of the structure and phase composition of an alloy of the Mg – Zn – Zr – Y system in cast, deformed and heat treated states are presented. It is established that the primary intermetallic phases are preserved in the alloy after deformation and heat treatment. Yttrium is shown to be an active phase-forming element just like zirconium. It is proved that the atoms of admixtures (such as H, N and O) in the Mg – Zn – Zr – Y alloy participate in the formation of the phase composition. The low anisotropy of the main mechanical properties of extruded bars from the alloy is explained.