The system (1 - x)NiIn2S4 - xNiIn2Se4 has been investigated by X-ray powder methods. The subsolidus phase diagram is constructed in the temperature interval 600 -1000 °C. The spinel type NiIn2S4 exhibits a phase width up to the composition NiIn2S2Se2 at 900 °C and NiIn2S2.8Se1.2 at 600 °C. A new layered compound is formed for 0.7 ≥ x ≥ 0.5 at 600 °C and for x = 0.6 at 900 °C which crystallizes in the MgAl2S4-type with a = 392.4 and c = 3739.6 pm (x = 0.6) for the hexagonal cell.