This volume comprises the compounds of thorium with the 4th main group elements silicon and germanium and all 5th main group elements except nitrogen. On the subject of ternary and polynary compounds containing oxygen, only those compounds with phosphorus are included (e.g., phosphates, hypophosphates, or phosphinates). Similar compounds of the other ele- ments like silicates, arsenates, etc. were already treated in "Thorium" Suppl. Vol. C 2, 1976. Most of the compounds in the different systems treated in this volume are of scientific interest because of their special physical and chemical properties. On the other hand, there are also many compounds wh ich are of specific technological interest, mainly in the nuclear field. Refractory compounds like Th Si or Th P have potential interest as nuclear fuel for special reactors due to their physical properties like good heat conductivity and their chemical stability, e.g., against the corrosion of cladding alloys for nuclear fuel elements. Due to the present situation in the further development of the thorium nuclear fuel cycle, their large potential has not yet been fully investigated. For most of the binary and ternary phases in these systems, we presently have good knowledge about the preparation of the compounds and their physical and chemical properties. But nevertheless, there are still a lot of open questions concerning phase equilibria, solid solutions, and homogeneity of some of the phases described in this volume.