A method has been developed for determining the depth profile of Fe+2and Co in cobalt-treated iron oxides non-destructively using X-ray Photoelectron Spectroscopy (XPS). Cobalt-treated oxides for magnetic recording were prepared from iron oxides containing various levels of Fe+2. The surface and near-surface layers of these oxides were analyzed by XPS. The Fe and Co 2p and 3s XPS spectra were resolved into multiplet splitting and shake-up components consistent with theoretical calculations and spectra for standard materials. The atomic ratios Co/Fe and Fe+2/Fe+3were then calculated for the surface and near-surface layers, and compared with those for the bulk sample in order to determine the Fe+2and Co depth profile. It was found that high Fe+2contents in the near-surface layers and cores of the particles increase diffusion of Co from the surface toward the core. This deeper penetration of cobalt is accompanied by increases in coercive force, magnetic moment, and tape conductivity, but also by higher magnetostriction, poorer print-through, and a broader distribution of particle Hc.