It is most preferred that the terminal hydroxy protecting group D in the resulting phosphorothioate oligonucleotide is removed in a step separate from and after the removal of further protecting groups, such as acetyl, benzoyl, phenoxyacetyl, 4-tert-butylphenoxyacetyl, N,N-dimethylformamidinyl, N-methylpyrrolidin-2-yliden, succinyl, 2-cyanoethyl and similar protecting groups that can be removed in