Tetrasulfurtetranitride, S4N4 reacts with (2‐pyridylamino)‐diphenylphosphine in MeCN at room temperature to form the cyclotrithiazene (NC5H4NH)‐Ph2PNS3N3 (1) in good yield. By contrast, the cyclophosphathiazenes Ph2PS2N3 (2) and 1,5(Ph2P)2S2N4 (3) are isolated from the same reaction mixture under reflux conditions. In solution, compound 1 is found to be transformed into 2. The reaction of S4N4 with (2‐pyridylamino)phenyl(dicyclohexylamino)phosphine in MeCN at room temperature affords Ph(DCA)PS2N3 (4) (DCA = dicylohexylamino) as the only reaction product. This on treatment with norbornadiene produces the addition product Ph(DCA)PS2N3·C7H8 ( 5 ). The structure of 4 has been established by X‐ray diffraction. Its PSN ring adopts a skew boat conformation with SN bond lengths varying from 1.574(4) to 1.606(4) Å. The mean value of the endocyclic PN bonds amounts to 1.618(3) Å. Copyright © 1994 VCH Publishers, Inc.