While not reported in the literature, by employing highly oriented, semi-insulating, thin-film CdS as testmaterial, we present the observation of the intrinsic photoluminescence Stokes shift in a semiconductor. Analysis of Raman peaks, transmission, reflection, and photoluminescence shows that during band-to-band emission at room temperature one-to-two longitudinal optical phonons with energy of 39.8 meV are emitted. The result is confirmed by the theoretically expected Huang-Rhys factor. (C) 2014 Elsevier B.V. All rights reserved.