The chemistry of P-N compounds is characterized by the existence of polymeric compounds. Many derivatives of the six membered ring system (NPCl(sub2)) and of the eight membered system (NPCl(sub 2))(sub 4) are known Therefore, the reaction of PCl(sub 5) with NH(sub 4)Cl has been studied The intermediates of this reaction could be isolated. In the re port a short review of useful substances derived from the trimeric and the polymeric phospho-nitrilic chlorides is given. There are also four membered P-N ring compounds. These four membered ring systems their preparation and their properties e.g. their structure, are described. Polymeric substances containing such a four membered ring system are formed with aromatic dihydroxy compounds and also with Cr(CO)(sub 6). Some four membered ring systems may be condensated. A compound with 3 four membered P-N ring systems was formed, in which the 3 ring systems are connected by a P-Cl bridge. There are also such ring systems in which 1 or more phosphorus atoms are substituted by S02 groups. Special attempts have been made to synthesize thermostable and flame resistant materials on the basis of P-N-C compounds. Therefore the reaction of guanidine and PCl(sub 5) was studied Other compounds were prepared by the reaction of PCl(sub 5) with cyanamide and with dicyandiamide, e.g. phosphatriazines could be prepared. New hetero cycles with four and five membered P-N ring systems were obtained by the reaction of oxamide and PCl Such ring systems can be connected with each other. Finally a very promissing product was obtained by the reaction of PCl and Liquid ammonia.