A solution phase synthesis method for preparing an oligonucleotide, wherein at least some of the reagents are solid supported. The method suitable for large-scale synthesis comprises coupling a protected compound with a nucleotide derivative having a protection group in the presence of a solid supported activator to give an elongated oligonucleotide with a P(III)-internucleotide bond; optionally processing the elongated oligonucleotide by capping by reaction with a solid supported capping agent and/or by oxidizing or sulfurizing by reaction of the oligonucleotide with a solid supported oxidizing or sulfurization reagent; and removing the protection group. The coupling may include reacting a 3'-protected compound of formula: ##STR00001## with a nucleotide derivative having a 5'-protection group, or reacting a 5'-protected compound of formula ##STR00002## with a nucleotide derivative having a 3'-protection group.