The structural role of aluminum upon replacement of lithium cations by sodium cations in the (0.8-x)LiF · xNaF-0.2Al(PO₃)₃ glasses is investigated by X-ray photoelectron and IR spectroscopies. It is shown that the equimolecular substitution of sodium for lithium is accompanied by changes in the coordination number of aluminum (Al(VI)→Al(IV)). The extrema observed in the concentration dependences E_σ=f(β_Na) and logσ=f(β_Na) (where β_Na = [Na]/([Na] + [Li])) are interpreted from the viewpoint of selective interaction between the glass components during the exchange reactions in the course of glass synthesis. It is demonstrated that the physicochemical properties of glasses are determined by the properties of different structural-chemical units produced during the glass synthesis, as well as by the concentration ratio of these units.