A quartz crystal microbalance (QCM) has been used to investigate mass transfer coupled with electron transfer through a hydrotalcite clay film. The electrode was prepared by casting an aqueous suspension of hydrotalcite (denoted as Mg-Al-HT) or hydrotalcite ion-exchanged with [Fe(CN)(6)](3-) (denoted as Mg-Al-HT-[Fe(CN)(6)])(3-). Adsorption studies using QCM and W-visible spectroscopic measurements revealed that [Fe(CN)(6)](3-) was incorporated as an ion-paired species with a counter cation. The adsorbed species on an electrode in contact with a 0.1 M NaCl solution was identified as a divalent ion-pair (Na[Fe(CN)(6)](2-)) while the adsorbed species in contact with a 0.1 M Na2SO4 or Na2CO3 solution was a monovalent ion-triplet (Na-2[Fe(CN)(6)](-)). Electrochemical QCM measurements on an electrode modified with Mg-Al-HT-[Fe(CN)(6)](3-) revealed that one Na[Fe(CN)(6)](2-) or Na-2[Fe(CN)(6)](-) ion was leached from a film when one or two [Fe(CN)(6)](3-) ions were reduced, respectively. The leaching of these anions took place in order to compensate the excess negative charge generated in a film. (C) 1998 Elsevier Science S.A. All rights reserved.