In order to reduce the number of sulphur vacancies in evaporated CaS:Eu and SrS:Ce electroluminescent thin films, Se-coevaporation can be performed. This process not only improves the brightness of the devices, but also induces considerable changes in the emission spectra. The shift of the electroluminescent spectra upon coevaporation was checked against semi-empirical molecular cluster energy calculations. In the case of SrS:Ce, these spectral changes are seen to be entirely due to the effect of electron-phonon interactions, while for CaS:Eu, only crystal field effects play a role.