The structural evolution of a binary mixture with nominal composition Co25Se75 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 CoSe2 phases are nucleated but there is also some amorphous selenium. For milling times between 30 and 38h, the volume fraction of the orthorhombic CoSe2 phase decreases and the volume fraction of the cubic CoSe2 phase increases. For milling times greater than 38h the cubic CoSe2 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.