Solid solution systems of Sm1-xYxS and SmS1-xAsx exhibit a composition- and temperature-dependent first- order electronic phase transition. The study of their optical properties and the analysis in terms of the scattering time of the conduction electrons has revealed strong resonant scattering in the collapsed metallic phase due to the configurationally mixed 4f level, degenerate with the Fermi energy EF. The measurements of the elastic constants indicate that none of the propagating modes become soft, although they show a discontinuity at the phase transition. The softening of the bulk modulus in the collapsed phase is attributed to zero energy excitations 4f6 → 4f5 5d1 at EF. The triggering mechanism of the electronic phase transition is found to be a pinning of EF within the configurationally mixed 4f level. © 1976.