Adsorptions of metallothioneins (MTs) onto thin mercury films at potentials more negative or positive than the point of zero charge (PZC) of mercury were quantified for the first time, using a novel electrochemical quartz crystal microbalance (EQCM) flow cell in conjunction with a simple flow injection system (FI-EQCM). The EQCM flow cell has a low internal volume (ca. 50 mL) designed for the investigation of biomolecular attachment to surfaces. Such a cell facilitates in-situ gravimetric measurements of MT adsorption processes at different potentials. It was found that the MT adsorption at a thin mercury film (TMF) proceeded via a combination of electrostatic interaction (through the attraction/repulsion between the positively charged MT molecules and the negatively/positively polarized electrode) and chemisorption (through the formation of Hg-cysteine bonds). The amount of the MT adsorption at –0.9 V, a potential more negative than the PZC of the TMF, was found to be greater than that attached via simple chemisorption (in an open-circuit configuration). The quantity of metals released by the MT adsorbates, measured by electrochemical inductively coupled plasma atomic emission spectrometry (EC/ICP-AES), was found to constitute ~3.8% of the total mass measured by FI-EQCM. The average number of cysteines per MT molecule involved in the Hg-cysteine thiolate formation was found to be 4.2 ? 1.8. Thus, it appears that about 1-2 metal ions (Cd2+) per MT molecule have been released off the TMF electrode since the stoichiometric ratio between the Cd2+ and the cysteines in a MT molecule is known to be 1/3. The quantitative measurements by the two hybrid electrochemical methods (FI-EQCM and EC/ICP-AES) help elucidate the electrode reactions of MT adsorbates at TMF surfaces and the MT metal transfer processes.