The disordering process in crystalline GeSb 2 Te 4 films has been studied by means of ion irradiation with 150 keV Ar+ ions. The effect of the interfaces and the role of the crystal microstructure has been investigated. The disordering path observed in a randomly oriented polycrystalline material with trigonal structure involves the transition to the disordered rocksalt structure (at fluence 7x10 13 cm-2) and then to the amorphous phase (at 1.5 x10 14 cm-2). In GeSb 2 Te 4 epitaxially grown on Si (111) the formation of the disordered rocksalt phase occurs at much higher fluence (3x10 14 cm-2) and it is preceded by the conversion of the stable phase into the ordered rocksalt structure (at 5x10 13 cm-2), with the formation of ordered vacancy layers, associated to a local variation of the stoichiometry. Even by increasing the fluence up to 3.5 x10 14 cm-2, the films remains mainly crystalline. The observed behaviour has …