A process for the sysnthesis of oligonucleotides using phosphoramidite chemistry is provided. The process employs as activator a 1,1-dioxo-1,2-dihydro-1.lamda..sup.6-benzo[d]isothiazol-3-one, preferably in the presence of an organic base. The 1,1-dioxo-1,2-dihydro-1.lamda..su- p.6-benzo[d]isothiazol-3-one is represented by the following structural formula: wherein p is 0 or an integer from 1 to 4; X.sup.7 is O or S; R for each occurrence is a substituent, preferably each independently, a halo, a substituted or unsubstituted aliphatic group, --NR.sup.11R.sup.12, --OR.sup.13, --OC(O)R.sup.13, --C(O)OR.sup.13, or cyano; or two adjacent R groups taken together with the carbon atoms to which they are attached form a six membered saturated or unsaturated ring; R.sup.11 and R.sup.12 are each, independently, --H, a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted aralkyl group; and R.sup.13 is a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted aralkyl group. Preferred organic bases are pyridine, 3-methylpyridine, or N-methylimidazole.