Elastic constant measurements under hydrostatic pressure show that, as semiconducting SmS approaches its type O isostructural transition from the larger-volume side, the bulk modulus decreases but not in the catastrophic manner characteristic of the intermediate-valence Sm_1-xY_xS alloys near the transition. In both the semiconducting SmS and the intermediate-valence metallic-state Sm_1-xY_xS alloys the contribution from cubic terms to the elastic strain energy is small.