A new selenium-transferring reagent, 3H-1,2-benzothiaselenol-3-one (1), has been developed for the conversion of nucleoside H-phosphonate and nucleoside H-phosphonothioate diesters into the corresponding seleno analogues. The reagent is soluble in common organic solvents and shows an enhanced rate of selenium transfer compared to elemental selenium or potassium selenocyanate. Selenization with the reagent 1 is stereospecific and most likely occurs with retention of configuration at the phosphorus centre.