AbstractA theory is given of effective dipole—dipole interactions between two identical substitutional guest molecules at large intermolecular distance. An interaction theory for interstitial impurity molecules was developed in [3]. A two‐level scheme for the guest and host molecules is assumed. Local field corrections are discussed. It is shown that the dipole—dipole interaction of the guest molecules in a crystal is the interaction of the effective dipole moments. The values of such effective dipole moments differ from their values in vacuum by a factor which is expressed both by the dielectric constant at the impurity transition frequency and the microscopic characteristics of the m