High solid-solubility Co 15Cu 85 alloys have been prepared by melt spinning and submitted to isothermal and anisothermal annealing to obtain granular alloys. The X-ray diffraction (XRD) patterns were measured to investigate the decomposition of supersaturated solid solution induced by annealing. The atomic diffusion and structural evolution during the heat treatment were investigated. In view of the problems limiting their application, the high-field magnetization curves were measured. By a fit to the high-field magnetization curves, the granular alloys are found hard to be magnetically saturated at the early stage of nucleation and growth. The magnetization behavior was correlated to the annealed-induced structural evolution and also to the magnetoresistance effect.