Tetrazole catalyzed reactions of a series of phosphoramidites, 5′ -O- DMTdT-3′-O-P(OR ¹)NRNR22 (1a-h), with 3′ O-SiBu ^tPh ₂-6-N-benzoyl-dA (2a) in acetonitrite solution have been studied. It is found that the coupling rate depends very much on whether tetrazole is added before or after 2a, and that dialkylammonium tetrazolide salts are inhibitors. These and other facts are evidence that the reactions are subjected to nucleophilic catalysis by tetrazole, in addition to acid catalysis. The rate variations with phosphorus substituents of 1a-h are NEt ₂ > NRr12 > N(CH ₂CH ₂)O > NMePh, and OMe > OCH ₂CH ₂CN > OCHMeCH ₂CN > OCMe ₂CH ₂CN >> OC ₆H ₄Cl. The inhibitor properties of dialkylammonium tetrazolides have practical consequences for the efficiency of DNA syntheses, when in situ prepared phosphramidites are used; the same would apply for segmented simultaneous syntheses or syntheses where recycling is performed.