In order to qualify the micro-corrosion analysis of magnetic recording disks by time-of-flight secondary ion mass spectrometry (ToF-SIMS), we investigated the information depth of Co+, estimated from the depth profiles obtained by dual-beam ToF-SIMS. On the carbon overcoat of 4 nm thickness, the Co+ intensity originating from the underlying magnetic layer was estimated as 0.85% of Co+ intensity at the interface, using lambda = 0.84 nm for the analytical conditions used. It was not negligible because Co+ intensity at the interface was significantly enhanced by the presence of the interfacial oxidation. Thus not only Co migrated to the surface but also Co contributing from the magnetic layer may be detected by ToF-SIMS in the case of thinly overcoated disks. (C) 2002 Elsevier Science B.V. All rights reserved.