Elemental [35S]sulfur was shown to equilibrate with the sulfur of thiophosphoryl chloride when these materials are heated together. This isotopic exchange reaction is the basis of a convenient, microscale synthesis of high specific activity [35S]PSCl3. [35S]Thiophosphoryl chloride is otherwise not commercially available except through custom synthesis. The labeled thiophosphoryl chloride was used in a novel procedure for the preparation of [35S]adenosine 5'-phosphorothioate. This isotopic exchange method should find wide application in the synthesis of many radiolabeled thiophosphoryl esters which utilize PSCl3 as the source of the thiophosphoryl group.