The title systems have been investigated by powder X-ray diffraction methods on quenched samples. In the system CuIn5S8-xSex two new compounds with layered structures have been detected. Samples quenched from 800 degreesC crystallise for 4 less than or equal to x less than or equal to 5 hexagonally with a = 395.81 and c = 3208.6 pm (x = 4) while samples quenched from 650 degreesC for 6 less than or equal to x < 7 are hexagonal as well but with a = 399.57 and c = 1615.5 pm (x = 6). In the system AgIn5S8-xSex no compound crystallising in a layered structure is obtained. AgIn5S0.8Se7.2 crystallises in a Wurtzite like structure with a = 408.4 and c = 667.0 pm. The spinel type phases CuIn5S8-xSex and AgIn5S8-xSex are stable up to x < 2 (800 degreesC) and x less than or equal to 5.6 (700 degreesC), respectively. The silver containing spinels decompose above 800"C into two spinel type compounds. At 1000 degreesC this decomposition is observed for 2.4 less than or equal to x less than or equal to 5.6.