We propose a novel single stranded DNA (ssDNA) immobilization process for improving sensitivity and reproducibility of DNA microarrays. We immobilized 50 mer 5'-amino-modified oligonucleotides by forming a covalent bond between the oligonucleotides, and phenylisothiocyanate groups tethered on the surface. We used a solution containing a phase-transfer catalyst, cetyltrimethylammonium bromide (CTAB). The amount of the immobilized oligonucleotides was estimated from fluorescence intensity of Cy5 dyes, which were attached to the oligonucleotides. The surface density of the immobilized oligonucleotides was also measured using X-ray reflectometry and surface plasmon resonance. We found that the addition of CTAB allowed us to increase the density and uniformity of the immobilized oligonucleotides, as well as to decrease non-specific adsorption of the oligonucleotides. The highest density and uniformity were obtained when the concentration of CTAB was nearly below the critical micelle concentration (CMC). At the concentration level close to the CMC, a CTAB monolayer formed on the surface can attract the negatively charged oligonucleotides effectively because of the positive charge of CTAB. Moreover, the CTAB layer can enhance encounter rates between the hydrophilic oligonucleotides and the hydrophobic surface with the phenylisothiocyanate groups. However, at a higher concentration level above the CMC, the density of the immobilized oligonucleotides decreases because the CTAB micelles can trap the oligonucleotides in the solution. Finally, we obtained higher hybridization signals on oligonucleotides immobilized with the CTAB solution. [DOI: 10.1380/ejssnt.2005.511]