Abstract In the realm of metal oxides perovskite-related phases occupy a niche of increasing importance. Reasons for this development may be characterized as follows: the structural framework is not made up of a close oxygen packing with metal cations occupying tetrahedral and octahedral interstitial positions as in corundum or spinel type oxides, but of a mixed metal/oxygen close packing. The general formula ABO3 reflects this fact clearly: in an AO3 cubic close packing small B metal cations are accommodated in oxygen octahedra. In this structural framework many different metal combinations may be stabilized. In many cases the stabilization of unusual formal oxidation states of the present metal cations is observed. Depending on the actual metal cation combinations, the chemical reactivity and the physical properties vary within a broad spectrum. In addition and as a rather specific phenomenon, the oxygen stoichiometry is variable within a considerable range while the basic structural framework, is conse...