The adsorption processes of oligonucleotides onto gold substrates have been investigated in aqueous phosphate buffer solutions using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) in conjunction with the quartz crystal microbalance. The hybridization of a thiol-modified, single-stranded oligonucleotide (HS-polyA), anchored to a gold surface via the thiol group, with its complementary sequence (polyT) has been observed and characterized via XPS and AFM techniques, which provide semiquantitative information about the amount of the material deposited and its surface morphology, making possible the monitoring of the hybridization process. The use of these techniques represents a complementary approach to those generally employed, such as spectrophotometry and surface plasmon resonance.