Polycrystalline samples of AgGa(Se1−zTez)2alloys were prepared by a melt‐and‐anneal technique, annealing at 650 °C for up to 4 months being required to give good equilibrium conditions. It was confirmed that single phase solid solution occurs at all values ofzand values of lattice parametersa,c, andc/awere determined from Guinier–Haegg powder photographs. Line intensities were obtained from diffractometer measurements and, hence, values of the anion displacementudetermined as a function ofz. Comparison of these experimental data has been made with the expressions proposed by Abrahams and Bernstein and by Jaffe and Zunger. The values ofufrom the former equation show rather poor agreement, particularly at the AgGaSe2end of the composition range. However, the equations of Jaffe and Zunger, used with Phillips radii, give very good agreement for bothuandabut poorer agreement forcandc/a. The optical energy gapE0is found to show a parabolic variation withz. Comparison with seven similar alloy systems shows that theE0versus composition curve is bowed for mixed‐anion alloys but practically linear for mixed‐cation alloys.