The structure and stability of Al overgrowths on platinum have been studied by LEED, STM, Auger spectroscopy and photoemission. Annealing dendritic Al islands on Pt{111} to 500 K promotes intralayer Al adatom diffusion across the substrate terraces. Evaporation from smaller Al dendrites results in island densification and coalescence, accompanied by Al/Pt intermixing at ascending steps. Simultaneously, triangular vacancy islands appear that decorate the step edges of the platinum surface. This process plays an integral part in the intermixing of Pt and Al at step sites. Metastable compact islands of the Al overgrowth persist up to 800 K, above which temperature interlayer transport commences, resulting in the formation of an ordered (2 x 2) Pt3Al surface alloy. Thus, a low-temperature quasi-one-dimensional alloying mechanism is succeeded by a place exchange mechanism at higher temperatures. The (2 x 2) alloy, formed for Al coverages above 0.5 ML, remains stable up to similar to 1000 K, at which point, complete Al dissclution occurs, though some Pt vacancy islands persist at the surface. (C) 2000 Elsevier Science B.V. All rights reserved.