For the first time, a demixing phenomenon is observed in a liquid binary metal-silicon system. Combined results from high-temperature differential thermal analysis and adiabatic calorimetry allow a determination of the enthalpy of phase transformation and the heat capacities in the solid and liquid states; furthermore, the equilibrium phase diagram around the ordered CoSi2 compound is corrected. The maximum of the liquidus curve lies only 8 K above the invariant transformation of the crystalline CoSi2 into two immiscible liquids.