Exact analytic expressions are obtained for Green’s functions of a linear chain with a diatomic unit cell containing an isotopic substitutional impurity. An infinite chain as well as a semi-infinite chain with an impurity at the free end are considered. The total change in the phonon spectrum and the spectral densities of vibrations of the impurity and its nearest neighbors are investigated. The conditions for the formation and characteristics of local and gap vibrations are analyzed. A general method of obtaining exact solutions by using Jacobi matrices is proposed for a wide class of one-dimensional systems. ©1998 American Institute of Physics.