Surface compositions and annealing effects for ultrathin Co/Ir(111) films influenced by Si capping layers have been investigated. On the surface of Co/Ir(111) films, Si adatoms are chemically active in the formation of Co-Si interfaces. Low-energy electron diffraction observations reveal that Si-Co interfaces is disordered in the long range. From sputtering profiling measurements, alloy formations occur at the interfacial layers, while pure cobalt remains on the Ir(111). After depositing submonolayer silicon on Co/Ir(111), the relative easy axis of magnetization in the polar geometry is mainly attributed to the positive interface anisotropy of the Co/Ir interface. The formation of nonmagnetic Co-Si compounds plays a dominant role in reducing the effectiveness of magnetic layers. Below 500 K, the interdiffusion of Si into the underlying Co layer occurs. Further interaction of Si atoms with Co layers and interdiffusion at the Co-Ir interface are observed at higher temperatures. This causes no detectable magnetic hysteresis after annealing up to 800 K. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3222909]