For DNA chip analyses, oligonucleotide quality has immense consequences for accuracy, sensitivity and dynamic range. The quality of chips produced by photolithographic in situ synthesis depends critically on the efficiency of photo-deprotection. By means of base-assisted enhancement of this process using 5'-¿2-(2-nitrophenyl)-propyloxycarbonyl-2'-deoxynucleoside phosphoramidites, synthesis yields improved by at least 12% per condensation compared to current chemistries. Thus, the eventual total yield of full-length oligonucleotide is increased more than 10-fold in the case of 20mers. Furthermore, the quality of every individual array position was checked quantitatively after synthesis. Subsequently, the quality tested chips were used in successive hybridisation experiments.