⁵⁷Fe Mössbauer spectra of (Mg,Fe)S solid solutions (niningerite) quenched from 1,000° C exhibit a singlet only within the composition range 8.4 to 50.0 mol. percent FeS. The isomer shift decreases progressively with increasing FeS content (from 0.99 to 0.95 mm/s) and extrapolates to a value of about 0.9 mm/s for FeS (halite structure), in close agreement with the isomer shift for FeS (troilite). Line width increases progressively with increasing FeS content (from 0.32 to 0.47 mm/s) and with decreasing temperature. The broadened singlet spectrum is interpreted as an unresolved composite quadrupole split doublet, which is attributed to local site distortion by next-nearest neighbor cation substitution. The ground state splitting is estimated to be 40 cm^–1 in (Mg_0.5Fe_0.5)S. The next-nearest neighbor effect in niningerite is anomalously small compared to that observed in spinel and (Mg,Fe)O compositions.