The magnetic properties of (FeIn2S4)1 − x (CuIn5S8) x single-crystal alloys are studied in the temperature range 4–300 K and in the field range 0–14 T. It is established that almost all of the alloys are paramagnetic materials at temperatures down to the lowest temperatures (∼5 K). It is shown that the ground magnetic phase state of the alloys is the spin-glass state with the freezing temperature steadily increasing with increasing content of Fe2+ cations. The most probable causes and mechanism for formation of the magnetic state of (FeIn2S4)1 − x (CuIn5S8) x single-crystal alloys are discussed.