Mass transport in clay films containing [Os(bpy)(3)](2+) cations was investigated by EQCM and crystal impedance spectroscopy. Admittance measurements on 10 mu g clay films exchanged with [Os(bpy)(3)](2+) show no change in the width or height of the conductance peaks before and after potential scans or potential steps. The [Os(bpy)(3)](2+) exchanged films could be considered to be rigid with no change in their viscoelastic properties, and the shifts in resonant frequency in the EQCM measurements interpreted as mass changes. In an electrode coated with a 8.5 mu g clay film, oxidation of the adsorbed cations resulted in a large increase in frequency, corresponding to a decrease in mass. The mass per mole of electron transferred (MPE) was -205 g/mol, or about one third of the weight of one [Os(bpy)(3)](2+) ion. This was consistent with the ejection of one [Os(bpy)(3)](3+) from the film for each three [Os(bpy)(3)](2+) cations oxidized. The ratio of cathodic charge to anodic charge of 0.7 was also consistent with the loss of one third of the oxidized [Os(bpy)(3)](3+) ions. The mechanism of charge neutralization was dependent of the weight of the clay films. Oxidation of [Os(bpy)(3)](2+) in a 33 mu g clay film resulted in a small decrease in frequency, corresponding to an increase in mass. The MPE, +50 g/mol, was consistent with charge neutralization by adsorption of sulfate anions from the electrolyte. (C) 2005 Elsevier B.V. All rights reserved.