First examples of phosphiniminocyclotrithiazenes containing a primary amino group, (Ph)(R)(1PrNH)PN-S3N3 [R=(i) Ph, I and (ii) DCA (dicyclohexylamino), II] have been isolated from room temperature reactions of S4N4 with the corresponding phosphine in acetonitrile. Single crystal X-ray structure of compounds I and n reveal very similar endocyclic structural features, but for the deviation of tricoordinate sulphur [S(1)] from the mean plane of the ring. The primary amino group [(CH3)2CHNH] binds equally strongly in I and H, but the N-H bonds show differences in both bond length and orientation with respect to the adjacent P-N bond. Heterocycles I and n show different sensitivities for ring conversion to cyclophosphathiazene.