Phonon dispersion has been measured in an intermediate-valence-state system. The phonons of intermediate valence Sm0.75Y0.25S are vastly different from those of the SmS and YS constituents. The LA phonons are unusually soft compared to the TA and have anomalous width, particularly for the  direction. The LO phonons lie below the TO. Temperature-dependent measurements show that the anomalous phonon properties change with changing valence. © 1978 The American Physical Society.