Fully-protected phospholriester derivatives of (Tp)₃T, (Tp)₇T, (Tp)₁₅T, and (Tp)₃₁T (18a; n= 2, 6, 14, and 30, respectively) were obtained in satisfactory yields by block synthesis. In all cases, stoicheiometric quantities of two starting materials (18b and c), each of which contained the same number of thymidine residues, were used. Phosphorylations were carried out in two steps with phenyl dihydrogen phosphate (16) and 2,4,6-tri-isopropyl-benzenesulphonyl chloride (7b) in pyridine solution. Fully-protected TpT. (Tp)₃T, (Tp)₇T, and (Tp)₁₅T (18a; n= 0, 2, 6, and 14, respectively) were converted into the corresponding unblocked oligothymidylic acids (21; n= 0, 2, 6, and 14, respectively) which were further purified by DEAE-cellulose or -Sephadex chromatography. In order to avoid terminal phosphoryl migration, the fully protected intermediates (18a) were converted into the corresponding tetrahydropyranyl derivatives (19) before they were submitted to alkaline hydrolysis to unblock the internucleotide phosphodiester linkages. The latter alkaline hydrolysis step led to ca. 3% cleavage per phospho-triester group and thus the phenyl protecting group or the present unblocking procedure is unsuitable for the preparation of oligonucleotides containing more than ca. 20 nucleotide residues.