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Co ultra-thin films deposited on Pt(111) are analysed in detail, using LEED, Auger spectroscopy and x-ray diffraction. The growth is quasi layer-by-layer up to about 3 ML. Co grows as islands, in incoherent epitaxy with the Pt substrate; the corresponding satellites are observed even for coverages as low as 0.12 ML. The Co in-plane parameter is very close to its own bulk value. For thicker films, deposited at room temperature, face centred cubic (fcc), twin fcc and hexagonal close packed (hcp) fractions are present.Using x-ray diffraction, we followed in real-time the transformations of the deposited film upon annealing. The role of the Co film structure (which depends on the film thickness) is predominant. For 'thick' films ( 6 ML) a sharp transition occurs around 670 K: below this temperature, the film is mainly hcp so that it allows little Pt incorporation, while above it turns to a very homogeneous fcc alloy of composition close to Pt 60 Co 40 . Segregation phenomena were analysed and found similar to those occurring at the surface of the corresponding bulk alloys.

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