A pseudo-unit-cell model is utilized to calculate some lattice-dynamical properties of a mixed diatomic lattice of the form AB1-xCx, where 0x1. The pseudo unit cell is formed by A, (1-x)B, and xC ions and the resulting mixed crystal lattice is a repetition of such cells. The lattice dynamics of both a linear and a three-dimensional mixed zinc-blende lattice have been studied. The lattice-dynamical model consists of central and noncentral short- and long-range interactions among ions of appropriate effective ionic charge. The model parameters are deduced from physical observables such as the elastic constants and the long-wavelength optical and impurity mode frequencies of the end systems when x0 and x1. Explicit calculations for the phonon dispersion and frequency distribution function have been performed for the linear mixed diatomic lattice and for ZnS1-xSex and GaP1-xAsx. © 1975 The American Physical Society.