d isothiazol 3 one to nucleoside phosphoramidite is in the range of from about 0 2 1 to 5 1 often from 0 25 1 to 4 1 preferably from about 0 3 1 to 2 1 for example about 1 1 then the support bound 5 u2032 deprotected nucleoside is contacted with the mixture for about 2 min to about 10 min preferably about 5 min if the terminal trivalent phosphorous linkage is to be oxidized after the coupling reaction is complete the solid support containing the nascent oligonucleotide is contacted with an oxidizing agent such as a mixture of i2 and water or a peroxide such as t butyl hydroperoxide in an organic solvent such as acetonitrile or toluene a mixture of i2 and h2o is a preferred oxidizing reagent when a mixture of i2 and water is used other water miscible organic solvents can also be present typically the solid support bound oligonucleotide containing trivalent phosphorous internucleotide linkages can be contacted with a solution of i2 in a solvent mixture of water an aprotic water miscible solvent and a base an example of a typical oxidation solution is about 0 05 m to about 1 5 m i2 in a solution of 2 80 20 water tetrahydrofuran lutidine vol vol vol the solid support is typically treated with the i2 solution for about 30 seconds to about 1 5 min alternatively the solid support bound nascent oligonucleotide can be contacted with a solution.