The T(z) diagram of the system (AgIn)2(1-z)(CdIn2)zTe4 is obtained from X-ray diffraction and differential thermal analysis measurements. At higher temperatures, there is a single phase solid solution across the complete diagram, the phase changing from a disordered defect zincblende to a partially-disordered tetragonal symmetry in the vicinity of z = 0.9. This high-temperature phase shows a eutectoid reaction at z = 0.65 and T = 490 °C giving the two tetragonal terminal phases α, with space group I42d corresponding to AgInTe2, and γ, with 14 corresponding to CdIn2Te4. At lower temperatures, the fields α and γ are separated by a relatively wide two-phase field (α + γ) which extends over the range 0.53 < z < 0.94. Values of room temperature optical energy gap E0 are determined from optical absorption measurements in the ranges of single phase behaviour and the form of the E0 versus z data curve is discussed. The form of this T(z) diagram is compared with that of related systems in which the terminal compounds have similar structure but differ in space group.