The isomer shifts of 149Sm have been measured in both the semiconducting and collapsed phases of samarium sulphide at room temperature. The isomer shift for SmS at atmospheric pressure (the semiconducting phase) is −0.72±0.04 mm/sec relative to a source of 149Eu in EuF3, with a linewidth of 2.20 mm/sec. The isomer shift in the collapsed phase was measured both in an Yttrium‐doped sample and in SmS as a function of pressure. The value in Sm0.77 Y0.23 S was −0.37±0.04 mm/sec, whereas that measured under a uniaxial stress of 11 kbar applied in a clamp device was −0.21±0.05 mm/sec. These results confirm the mixed valence state proposed for the non‐magnetic, metallic phase, and from the linewidth 2,1 mm/sec, it is deduced that the 4f5 5d→←4 f6 fluctuations are more rapid than 10−9 s. The decreasing isomer shift in the series SmX, X=S, Se, Te is inconsistent with the suggestion that there is configurational mixing in the semiconducting state of SmS which is absent in Sm Se.