Abstract The simple random field theory is presented for the description of the equilibrium properties of impurity systems with concentrational phase transition. The theory essentially differs from earlier appraoches, based on random mean field approximations, because it explicitly takes into consideration the thermal fluctuations of internal fields. The application of the theory to well-known model systems shows that it gives at least qualitatively true description of the impurity systems with concentrational phase transitions. The developed general formalism was used for the analysis of concentrational phase transition in highly polarized dielectrical crystals with dipole impurities. The experimental situations in KTaO 3 and other systems are discussed. The theory permits also to describe the equilibrium cooperative properties of the systems without long-range order (spin and dipole glasses in particular).