This study presents a polyviologen (PV) modified gold/quartz electrode to detect paraquat (MV 2ΓΎ ) by electrochemical quartz crystal microbalance (EQCM) method. Some electroactive viologens (V 2ΓΎ ) reside in the electropolymerization achieved in pH 4.2 Britton-Robinson (BR) buffer solution containing soluble viologen oligomer. Closely examining the EQCM and cyclic voltammetry (CV) results reveal that V 2ΓΎ binds in the PV film and is electroreduced to form its relatively hydrophobic cation radical (V ΓΎ. ), which may cause the anions and hydrated water originally incorporated in the PV layer to be expelled. Moreover, while the negative potential is scanned over ΒΉ0.55 V, the dissolved MV 2ΓΎ is reduced through V ΓΎ. to form its corresponding cation radical (MV ΓΎ. ) and then becomes a dimer with the bound V ΓΎ. among the PV layer. According to our results, such dimerization possesses good reversibility so that the resonant frequency of the subsequent PV modified Au/quartz electrode can return to its initial value under the potential scanning within an appropriate region (i.e., from 0 to ΒΉ0.7 V) and oxygen-free condition. Moreover, a linear calibration plot for detecting MV 2ΓΎ is also obtained in the range from 0 to 1.2 mM, having a detection limit of 0.1 mM.