X-ray photoelectron spectroscopy, we investigate the morphology of Co ultra-thin films grown at room temper- ature on Ir(111) and graphene/Ir(111) surfaces, a system of interest in view of novel nanomagnetism and spintronics systems. When Co/graphene/Ir(111) heterostructures are annealed at 570 K, Co intercalates below the graphene layer. Subsequent annealing to 770 K leads to Co and Ir intermixing and formation of a surface alloy extending over a thickness of about 2–3 monolayers.