Diastereomerically pure, isotopically labeled 5'- O -DMT-nucleoside-3'- O -(2-thio-4,4-"spiro"-pentamethylene-1,3,2-[ 18 O]oxathiaphospholane)s and -(2-oxo-4,4-"spiro"-pentamethylene-1,3,2-[ 18 O]oxathiaphospholane)s suitable for stereocontrolled synthesis of P-chiral oligonucleotide [ 18 O]phosphorothioates and [ 18 O]phosphates were synthesized. Obtained in that way "chimeric" oligonucleotide d[A PO A PS-Rp,Sp A PS-Rp , Sp A PS-Rp C PS(18O)-Rp G PS-Rp T PS-Rp T PS-Rp,Sp T PO T] was used for determination of the stereochemistry of hydrolysis by endonuclease from Serratia marcescens .