With the increase of magnetic storage capacity in view, sub grain decomposition in Co-Cr magnetic recording material is examined with the field ion microscope / atom probe (FIM/AP) on a nanometer scale. Concentration variations in sputtered 50 nm Co-20at% Cr and 3 μm Co-22at% Cr films are analysed. Layers with up to 50 nm thickness are sputtered directly onto tungsten FIM tips. FTM-tips of layers with up to 3 μm thickness are prepared by a lithography and etching technique. The measurements show decomposition on the scale of a few nanometers. The concentration amplitudes vary between 11 at% Cr and 42 at% Cr within 50 nm films. Within 3 μm films the concentration varies from about 10 at% Cr to 30 at% Cr. Furthermore, at grain boundaries 3–4 nm precipitates of about 40 at% Cr are observed. The observed concentrations depend on substrate temperature. The compositional inhomogeneity can be explained by a miscibihty gap in the hep phase solid solution which is induced by magnetism at low temperatures.