Homo-oligomer DNA strands were immobilized onto silicon/silicon dioxide electrodes using 3-aminopropyltriethoxysilane. These modified substrates were used as working electrodes in a three-electrode electrochemical cell. In-phase and out-of-phase impedances were measured in the range -1 to +1 V with respect to an Ag/AgCl reference electrode, with a superimposed 10 mV ac signal at frequencies of 20 and 100 kHz. Ex situ hybridization with complementary oligomer strands, performed at the surface of modified electrodes, is clearly reflected by negative shifts of about 100 mV in the flat-band potential of the semiconductor. Consecutive hybridization-denaturation steps show that the shifts are reproducible and the process is reversible. The in situ hybridization of complementary strands has also been observed with impedance measurements at Si/SiO2 substrates and with the use of a field effect device. The direct detection of hybridization with a field effect device was performed under constant drain current mode, and the corresponding variations observed for the gate potential during hybridization are in good agreement with the flat band potential shifts observed with the impedance experiments. Measurements made in the presence of noncomplementary strands demonstrate the selectivity of the device.