A solution of sulfur (0.1 M) and sodium sulfide (0.01M) in 3-picoline, referred to as polysulfide reagent, rapidly converts trialkyl and triaryl phosphite triesters to the corresponding phosphorothioate derivatives. Greater than 99.8% average stepwise sulfurization efficiency is obtained in the solid-phase synthesis of DNA and RNA phosphorothioate olgonucleotides via the phosphoramidite approach.