The growth of cobalt on Cu(lll) has been studied using a variable-temperature scanning tunneling microscope (STM). Al a deposition temperature of 150 K, one observes the growth of three-layer Co islands with one subsurface layer. The Co islands are surrounded by a brim of Cu. The distinction between Co and Cu is made by adsorption of CO which adsorbs only on Co at room temperature, resulting in a Co(lll)-(root 3 x root 3)R30 degrees-CO structure. After heating the surface, or depositing Co at higher temperatures, the Cu brims gradually disappear, and vacancy islands form in the Cu(lll) surface. The top-layer Co-Cu composition changes slowly at room temperature with Co being replaced by Cu on a timescale of similar to 1 h, consistent with earlier ion-scattering studies. The experimental findings are in accordance with ab-initio total-energy calculations showing the thermodynamically stable island configuration to be several cobalt layers capped with one copper layer. (C) 1997 Elsevier Science B.V.