Polycrystalline samples of (CuIn)x(AgIn)yCd2zTe2were prepared by a melt and anneal technique. Guinier x‐ray powder photographs were used to determine equilibrium conditions and lattice parameter values. Two single‐phase fields, with zinc‐blende and chalcopyrite structures, are found to exist in the diagram. Comparison of the experimental results with the expressions proposed by Jaffe and Zunger has been made. It was found that these equations, used with Phillips radii, give good agreement for parameterafor Cu‐rich samples but poorer agreement elsewhere and also forcandc/a. Optical absorption measurements were carried out on most of the samples to determine values of the optical energy gapE0at room temperature. These values ofE0are found to show a nonlinear variation withz. This variation is discussed in terms of the effects ofp‐dhybridization on the valence band and the change in crystal structure.