The present invention provides an improved phosphoramidite activator comprising a solution of 5-benzylmercaptotetrazole in acetonitrile and N-alkyimidazole. Further provided are improved methods of oligonucleotide synthesis wherein a phosphoramidite activator comprising a solution of 5-benzylmercaptotetrazole in acetonitrile and N-alkyimidazole is used to prepare internucleoside linkages.