The structural evolution of a binary mixture with nominal composition Co 2 5 Se 7 5 prepared by ball milling was investigated as a function of milling time using X-ray diffraction (XRD), differential scanning calorimetry and Raman spectroscopy. The XRD patterns show that for milling times up to 30h, orthorhombic and cubic CoSe 2 phases are nucleated but there is also some amorphous selenium. For milling times between 30 and 38h, the volume fraction of the orthorhombic CoSe 2 phase decreases and the volume fraction of the cubic CoSe 2 phase increases. For milling times greater than 38h the cubic CoSe 2 phase starts to decompose and a hexagonal CoSe phase begins to form. For long milling times, the XRD patterns show only the hexagonal CoSe phase.