The surface attachment and detection of DNA probes are essential in the design of nucleic acid-based biosensors. A new strategy for the covalent immobilization of single-stranded oligonucleotides on gold-covered planar supports is presented. Optimization of the surface density in the resulting DNA arrays permits a high hybridization efficiency to be achieved. Surface plasmon resonance and, for the first time, ATR-FTIR spectroscopy are used to follow in situ the oligonucleotide layer formation and the subsequent complementary strand hybridization. Such well-defined, covalently immobilized oligonucleotide arrays can find application in the development of novel DNA-based sensors for mutation detection and gene mapping as well as in studies of nucleic acid-ligand interactions.