Within both the phenomenological approximation and the framework of the principle of additivity of physical quantities of a multicomponent solid solution, including high-entropy alloys, the concentration dependences of the lattice parameter and the Debye temperature are calculated. The principle of additivity can be applied to these physical quantities, since they can be considered indirectly through the radius and mass of atoms as their own, but not collective characteristics of the individual components of the alloy. It is proposed to consider the integral and differential concentration coefficients as the quantitative measures, the values and signs of which allow us to establish the influence of each alloying element, which is represented alternately by all elements of the system, on the values of the lattice parameter and the Debye temperature. The 4–6-components’ systems based on Fe, Ni, Co, Cu, Cr, Al and Ti within the equiatomic approximation are analysed.