The adiabatic compressibility, P, of the immiscible liquid mixture 0.5 NaCl + 0.5 AgI in the range of immiscibility was investigated both experimentally and through the use of a model of charged hard spheres with different diameters. Experimentally, the compressibility was studied using the sound velocity values, u, measured by the pulse method, and the density quantities, p, which were determined using the hydrostatic weight procedure based on the relationship P = 1/u[1] p. We show that the compressibility of the upper phase decreases but that of the lower phase increases when the temperature rises because of the superposition of the intensity of the thermal motion of the ions and the change in the composition of the phases. The difference, ДР, in the magnitudes of the compressibility for the equilibrium phases decreases with temperature elevation. The temperature dependence of the compressibility difference is described using the empirical equation ДР = (Tc - J)0-442, which is close to the mean-field theoretical description. The results of the model calculations sufficiently reproduce the experimentally observed temperature dependence of the compressibility of the coexisting phases. However, note that the theoretically predicted critical exponent (1/2) is different from that determined experimentally by 13%. The results are discussed in terms of the chemical bond nature of AgI.