Taking advantages of psoralen photochemistry, we have developed a new method of immobilizing DNA on gold substrate surfaces. A psoralen derivative having an alkylamine function was synthesized, and was self-assembled on gold substrate surfaces in a combined use of a thiol-derivatized molecule, 3,3'-dithiobis(succinimidyl propionate) forming amide bonds on the surface. We found that by irradiating with long wavelength ultraviolet light (320-400 nm), DNA molecules added in the solution phase were covalently immobilized on the monolayer surface through the photoadduct formation of the psoralen molecules with the DNA nucleobases. The present method has its advantage that is applicable to native DNAs, no chemically modifying DNAs, in spite of its covalent immobilization principle. We have examined 12 mer synthetic oligonucleotide immobilizations and have found that the surface concentration thus attained was to be 20 pmol cm(-2), which is consistent with saturated surface coverage. Interestingly, the immobilization occurred double-stranded-DNA-preferentially; no immobilization for single-stranded DNAs. Characterization of the immobilization chemistry has been achieved using atomic force microscopic imaging, infrared absorption, X-ray photoelectron spectroscopy, electrochemistry, and quartz-crystal microbalance and their results were described.