Photodirected oligonucleotide synthesis uses either direct or indirect light-dependent 5'deprotection. Both have been reported to give lower stepwise synthetic yields than conventional methods. The deficiency appears to be due to incomplete deprotection at the, oligonucleotide 5'-position and, additionally in the case where photodirection is indirect and uses photogenerated photoacid to effect 5'-detritylation, the depurinating effects of strong acid. We have developed novel photosensitive-2-nitrobenzyl esters that on irradiation with near UV light generate alpha-chloro-substituted acetic acids, such as trichloroacetic acid, which are widely and successfully used in conventional solid-phase oligonucleotide synthesis. alpha-Phenyl-4,5-dimethoxy-2-nitrobenzyltrichloroacetate and alpha-phenyl-4,5-dimethoxy-2,6-dinitrobenzyltrichloroacetate showed appropriate photochemical characteristics and were used for photodirected synthesis of a variety of oligonucleotides, including (T)(5), TATAT, TGTGT, (T)(10), (AT)(5), (CT)(5) (GT)(5), and (TGCAT)(2) on a modified Millipore Expedite DNA synthesizer. The outcomes were compared with those obtained by use of directly added trichloroacetic acid (conventional synthesis). The stepwise yields for the two methods were essentially identical.