Glasses, crystals, and melts in the Zn2P2O7–M4P2O7(M= Li and Na) systems are studied by differential scanning calorimetry (DSC). Temperature dependences of the heat capacity and the enthalpies of melting of Zn2P2O7and Na2ZnP2O7crystals are determined. The thermodynamic characteristics obtained are used for calculating the liquidus curve for the Zn2P2O7–Na2ZnP2O7subsystem on the basis of the Schroder relationship with due regard for the temperature dependences of the enthalpy of melting for components and the nonideal behavior of melts. Reasoning from the Hruby empirical formula, the inference is drawn that the crystallization ability of lithium–zinc phosphate glasses is higher than that of sodium–zinc phosphate glasses. The radius of cooperative motion in the glass transition range for glasses in the Zn2P2O7–Na4P2O7system is estimated in the framework of the phenomenological thermokinetic fluctuation theory. It is assumed that an increase in the radius with an increase in the alkali oxide content is associated with an increase in the ionicity of bonds.