The heats of mixing of binary liquid alloys of copper with barium, europium and ytterbium in the copper-rich concentration region and the heats of mixing in the Cu-Dy system over the total composition range were measured by isoperibolic calorimetry. The heats of formation increase with increasing atomic number, i.e. in agreement with previous investigations for other Cu-Ln melts. The component interaction energy is found to increase along the row Ba, Eu, Yb, Dy. An apparent variation in the partial mixing enthalpies with atomic size factor which contributes positively to the alloy formation energy is observed. Model calculations of integral enthalpy for the Cu-Ba system based on the atomic size difference factor allow one to estimate the mixing enthalpy in the total concentration region.