Temperature dependence of the magnetic properties of ultrathin Fe/Cu3Au films (thickness <15 Angstrom) grown at roomtemperature (RT) have been investigated in UHV by means of magneto optical Kerr effect (MOKE). Thermal magnetic properties of this system have been investigated by monitoring the Kerr asymmetry (A(k)) versus temperature during thermal cycles of annealing of the films from RT to 400 K and subsequent cooling back to RT. For films with thickness below 10 Angstrom we observe an irreversible decrease of the measured Kerr signal upon such thermal treatment, while reversible behaviour gradually onsets as the film thickness is increased towards 15 Angstrom. The irreversible behaviour is ascribed to an irreversible morphological transformation undergone by the film upon annealing, which also influences its magnetism. The appearance at thickness above 10 Angstrom of reversible magnetic behaviour upon thermal treatment is explained taking into account the evolution of the structural properties of the Fe film: for thickness below 10 Angstrom the film is in a metastable fcc pseudomorphic phase, while nucleation of a more stable tetragonally strained phase onto the film onsets for larger thickness: the different magnetic properties and stability upon temperature of the two phases can lead to the observed experimental evidence. (C) 2003 Elsevier Science B.V. All rights reserved.