The enthalpy of formation of copper + silicon liquid alloys has been measured at 1370 K by direct reaction calorimetry. The enthalpy of formation is negative in the whole range of composition and has a minimum -(14.35 ± 0.3) kJ mol-1, for a mole fraction x(Si) = 0.25. From the thermodynamic behaviour of the melts, the assumption was made that the metastable deep eutectic between pure Cu and Si can be explained by chemical bonding around Cu3Si. © 1979.