Inelastic-neutron-scattering studies using triple-axis and time-of-flight techniques have been made in the low- and high-pressure phases of a polycrystalline sample of Sm154S. There is no observable change in the energy or width of the F07-F17 excitation of the 4f6 configuration between ambient pressure and 0.6 GPa. In the mixed-valence phase at high pressure, there is no evidence for well-localized excitations from either the F07-F17 (4f6) or the 7-8(H526, 4f5) configurations. These results suggest that the mixed-valence phase is homogeneous. Lattice-parameter measurements versus pressure at 85 and 298 K show no evidence for pretransitional softening and establish that the phase diagram in the P-T plane is reentrant. From the phase diagram and earlier heat-capacity measurements, the electronic entropy in the mixed-valence phase is estimated to saturate at 2R with increasing temperature.