The bacteriophage T7 induced DNA polymerase, consisting of the phage specified gene 5 protein associated with Escherichia coli thioredoxin, catalyzes the copolymerization of SP-dATP alpha S with dTTP, producing the alternating of polymer poly[dTs-A)] by a mechanism involving inversion of configuration at P alpha. Degradation of poly[d(5s-A)] by the nucleolytic action of E. coli DNA polymerase produced the dinucleotide pdTps-dA, whose configuration at the phosphorothioate diester was assigned as R by comparison of the phosphorus-31 nuclear magnetic resonance chemical shift (55.0 ppm downfield from H3PO4) with that of an authentic sample. Further degradation by alkaline phosphatase to Rp-dTps-dA (55.6 ppm downfield from H3PO4) confirmed the configuration. The stereochemistry provides no evidence of a double displacement mechanism.