The structure (size, shape, orientation) of CoPt alloy islands formed on NaCl(001) substrates was investigated using grazing-incidence small-angle X-ray scattering (GISAXS) combined with electron and X-ray diffraction and transmission electron microscopy. Co50Pt50 and Co25Pt75 islands formed at 473 K by co-deposition of Co and Pt species are oriented along the [001] direction, and their shapes are well described as truncated spheres with lateral sizes of around 4–5 nm. In contrast, using the same Co and Pt atomic fluxes, co-deposition at 673 K leads to the formation of strongly enriched Pt islands with larger sizes, and different orientations and shapes. The Pt enrichment is attributed to a large increase in the difference between the Co and Pt condensation coefficients with deposition temperature. Surprisingly, for the island assemblies formed at 673 K, the GISAXS patterns are dominated by the scattered …