The composition and structure of carbon overcoat layers on hard discs were studied systematically using secondary ion mass spectrometry (SIMS). The amounts of H, C, N, and O in the carbon overcoat were measured along with Co and Cr outdiffusion from the underlying magnetic layer. Two SIMS techniques were used, the "MCs+" technique in which the secondary ion intensity is linearly proportional to the concentration [M], and the "M2M3" technique in which the secondary ion intensities show quadratic and cubic dependence on [M]. The MCs+ technique was used to measure stoichiometry and thus characterize diamond-like carbon, hydrogenated carbon, and carbon nitride overcoat layers or sublayers. The M2M3 technique was used to provide structural information on the physical uniformity of the overcoat. The SIMS results are compared to those from other techniques, emphasizing the relative value and cost of information. The tribological issues affecting hard disc durability and wear are discussed.