New monomers, 5＇-O-DMT-deoxyribonucleoside 3＇-O-(2-thio-"spiro"-4,4-pentamethylene-1,3,2-oxathiaphospholane)s, were prepared and used for the stereocontrolled synthesis of PS-Oligos via the oxathiaphospholane approach. These monomers and their Zero analogues were used for the synthesis of "chimeric" constructs (PS/PO-Oligos) possessing phosphate and P-stereodefined phosphorothioate internucleotide linkages. The yield of a single coupling step is approximately 92-95%, and resulting oligomers are free of nucleobase- and sugar-phosphorothioate backbone modifications. Thermal dissociation studies showed that for heteroduplexes formed by [R-P]-, [S-P]-, or [mix]-PS/PO-T-10 with dA(12), dA(30), Or poly(dA), for each template, the melting temperatures, as well as free Gibbs＇ energies of dissociation process, are virtually equal. Stereochemical evidence derived from crystallographic analysis of one of the oxathiaphospholane monomers strongly supports the participation of pentacoordinate intermediates in the mechanism of the oxathiaphospholane ring-opening condensation.