Abstract: Following a short review of early studies of the valence band of tetrahedral oxyanions of types XO 4 n- some recent results of this field are discussed. High resolution XPS measurements of the valence band of phosphorus and sulphur oxyanions made possible a rigorous test of various theoretical models. For DV-X α cluster MO calculations, experimentally determined crystal structure information were used to set up realistic model clusters. In the case of the SO 4 2- cluster, the results of several model calculations (ab initio, DV-X α , hybrid models) are presented. From the comparison of these results a better understanding of the role of the contributions from different effects to the MO one-electron energies can be obtained.