P-32-labeled DNA oligomers were immobilized onto self-assembled monolayers formed by carboxy-modified alkylthiols on gold electrodes. The optimal conditions were evaluated by comparing of surface density of oligonucleotides measured for different immobilization conditions before and after desorption of physically adsorbed nucleotides. The results demonstrate that the physical adsorption depends strongly on immobilization conditions being typically 10 times higher than the chemical immobilization. Under optimal immobilization conditions, the surface density of oligonucleotides was more than 20% of the value calculated for a densely packed monolayer.