In CdS the interactions between the lattice and the electronic excitations which luminesce can be understood in terms of a Franck–Condon model. Calculations for the value of the electron – LO phonon interaction parameter S done by Toyozawa for a bound carrier are extended to include the free exciton, the bound exciton, and the bound to bound transition. Experiments showed that Ŝ for excitons varied inversely with exciton size in agreement with theory. Calculations which showed that [Formula: see text] for the bound to bound transitions varied with the donor–acceptor separation were verified by experiment.