The growth of copper on the hexagonally reconstructed Pt(100) surface has been studied in UHV by scanning tunneling microscopy (STM). The copper film grows anisotropically and initially almost in a bilayer fashion due to a high diffusion barrier at copper island edges and a reduced mobility on the Cu islands. The substrate reconstruction is lifted only locally by Cu island formation. This lifting leads to an ordered incorporation of platinum excess atoms into the first layer of the growing copper film: At 340 K, parallel, monatomically wide platinum chains are formed, whereas at 650 K, broader line structures appear in which the platinum atoms are again hexagonally arranged. Up to nine monolayers, the Cu films remain pseudomorphic with the Pt(100) substrate but undergo an fcc f bct transition beyond 4 monolayers.