Abstract Magnetic properties of polycrystalline solid solutions MnxMg1−xS are reported. From these data a critical concentration xc = 0.13 ± 0.01 was obtained, in accordance with calculations from series expansion of the mean cluster size for a f.c.c. lattice with exchange interactions up to 12 n.n. and 6 n.n.n. Magnetic measurements on single crystals of α - MnS show anomalies which are not observed in powder samples.