A method for preparing an oligonucleotide comprising the steps of a) providing a 3-protected compound having the formula: wherein B is a heterocyclic base R2 is H, a protected 2-hydroxyl group, F, a protected amino group, an O-alkyl group, an O-substituted alkyl, a substituted alkylamino or a C4'-O2'methylen linkage R3 is OR'3, NHR''3, NR''3R'''3, a 3'-protected nucleotide or a 3'-protected oligonucleotide, R'3 is a hydroxyl protecting group, R''3, R'''3 are independently an amine protecting group, b) reacting said compound with a nucleotide derivative having a 5-proctection group in the presence of a solid supported activator to give an elongated oligonucleotide with a P(III)-internucleotide bond c) optionally processing the elongated oligonucleotide with a P(III)-internucleotide bond by either or both of steps c1) and c2) in any sequence c1) capping preferably by reacting with a solid supported capping agent c2) oxidizing preferably by reacting the oligonucleotide with a solid supported oxidizing reagent d) removing the 5'-protection group.