The modified triester approach has been further improved and refined to the synthesis of defined sequences of deoxyribooligonucleotides. Improvements include arylsulfonyltetrazoles as faster and milder condensing agents, benzenesulfonic acid to avoid depurination during deblocking of trityl protecting groups and improved chromatographic procedures for purification of triester intermediates and purification of the final product containing 3′-5′ phosphodiester linkages. The efficiency of these new modifications in terms of yields and time has been illustrated by the synthesis of various defined sequences related to the lactose-operator region of E. coli, its analogues and two restriction enzyme recognition regions.