In bioelectric recordings, an electrode-skin impedance mismatch leads to a reduced common-mode rejection ratio (CMRR) of the amplifier. For this reason, the impedance of each individual electrode-skin contact is usually measured prior to a recording. The measurement circuit itself degrades the CMRR and is switched off during the bioelectric recording. In this paper, we present a new method that allows to monitor the electrode-skin impedance in a continuous way without reducing the CMRR of the amplifier. The new method is based on an additional common-mode signal that is superimposed on the bioelectric signal.