The enthalpies of mixing ΔH and excess volumes ΔV were analyzed in alloy formation from binary metallic eutectic systems. Three types of systems can be distinguished: I with ΔH > 0, ΔV > 0; II with ΔH > 0, ΔV ≤ 0; III with ΔH < 0, ΔV ≥ 0. The hypothesis is discussed that another yet unstudied group IV exists in which ΔH < 0, ΔV < 0.