The structure of the aluminide phase (AlxFeNi) in cast AA2618 has been investigated by means of electron probe microanalysis and transmission electron microscopy techniques. The AlxFeNi phase has been determined to have a C-centred monoclinic structure with lattice parameters a = 0.8673 nm, b = 0.9000 nm, c = 0.8591 nm, and beta = 83.504 degrees. It is also shown that the structural formula of the phase varies from particle to particle and is influenced by the aluminium content.