Sulphate–chloride glasses, doped with small quantities of cobalt(II), have been prepared with compositions varying from all-sulphate to all-chloride. The visible spectra indicate that the co-ordination of cobalt(II) undergoes a series of transitions, octahedral → pseudo-tetrahedral → octahedral → tetrahedral, as the chloride concentration increases. These changes are related to different ways in which sulphate behaves as a ligand and to the structure of the glass. A correlation with certain molten salt systems is discussed.