Abstract: The use of N-formamidino-5-amino-3H-1,2,4-dithiazole-3-thiones, 5-phenyl-3H-1,2,4-dithiazole-3-thiones, and derivatives thereof as novel, efficient sulfur-transfer reagents is disclosed. Sulfur transfer from these reagents to compounds containing a P(III) atom (e.g., triphenylphosphine, 5?-O-DMT-thymidine 2-cyanoethyl-(N,N-diisopropyl)phosphoramidite, and 5?-O-DMT-3?-O-levulinyl dithymidilyl 2-cyanoethyl phosphite), was studied in solution by 31P NMR and HPLC. The sulfur transfer from title compounds was also studied in the solid-phase synthesis of oligonucleotide phosphorothioates by phosphoramidite methods. In this application, the efficiency of the sulfur transfer reaction for 2?-deoxyoligonucleotides was better than 99.5%. The novel sulfurizing agents are synthesized, at low cost, using simple chemical methods.