The reaction of 1,3-dimethyl-1,3-diaza-2-chloro-5,6-benzo-1,3,2-phosphorinan-4-one 1 with N-trimethylsilylacetamide 2 and 2-methylpiperidine hydrochloride furnished the P-substituted derivatives 3 and 4. The reaction of 1 with trimethylbromo- and trimethyliodosilane yielded the 2-bromo- and 2-iodo- substituted compounds 5 and 6. 1-Methyl-3-(2-chloroethyl)-1,3-diaza-2-chloro-5,6-benzo-1,3,2-phosphorinan-4-one 8 was prepared in the reaction of N-methyl-N'-(2-chloroethyl)-anthranilamide 7 with phosphorus trichloride. The reaction of 8 with N-trifluoromethyl-3-nitrophenylamine 9 or with sodium fluoride led to the 2-fluorosubstituted product 11. From the unusual reaction of 8 with 9 in the presence of triethylamine N-difluoromethylene-3-nitrophenylimine 10 and the intermediate product, triethylamine hydrofluoride, were isolated. A mechanism of formation of 11 is proposed. The reaction of 8 with N-trimethylsilylacetamide 2 and bis-2-chloroethylamine hydrochloride furnished the expected substitution products 14 and 15, involving the 5,6-benzo-1,3,2-phosphorinan-4-one ring system. The characterization of 3-6, 8, 11, 14 and 15 rests, especially, on their 1 H-, 13 Cand 31 P-NMR spectra, and their mass spectra.