A reagentless electrochemical impedance biosensor for detection of peanut protein Ara h 1, one of the allergenic proteins found in peanuts, has been demonstrated using an Au substrate onto which an antibody film has been immobilized. Following initial stabilization of the self-assembled monolayer (SAM) through which the antibody is immobilized, the biosensor substrate exhibits stable impedance spectra at different stages of substrate preparation. By fitting the impedance spectra to a Randles equivalent circuit, one can demonstrate that the charge-transfer resistance (R(ct)) increases and the differential capacitance (Cd) decreases with increasing concentration of Ara h 1, although R(ct) exhibits greater sensitivity. The detection limit of this reagentless biosensor is estimated to be less than 0.3 nM. Assuming a Langmuir adsorption isotherm, the dissociation constant of the peanut protein Ara h 1 and its antibody can be calculated as 0.52 nM from the variation in differential capacitance with Ara h 1 concentration.