The time dependence of the voltammetric waves of [Fe(bpy)3]2+ adsorbed in clay-modified electrodes (CMEs) differed greatly from those of [Ru(bpy)3]2+ and [Os(bpy)3]2+. The currents obtained with the ruthenium and osmium cations were essentially constant in the first 2 h that the CME spent in the blank electrolyte. For [Fe(bpy)3]2+, the maximum currents were twice as large. After a sharp rise in the first few scans, they decreased rapidly to less than half of their maximum values after 40 min. The decrease was more rapid when the potential was scanned continuously or when the pH of the electrolyte was increased. Coulometry shows that a larger fraction of the adsorbed [Fe(bpy)3]2+ cations were oxidized and that they were oxidized much more rapidly than the other two cations. The unique behaviour of [Fe(bpy)3]2+ is attributed to its dissociation in the CME. UV-visible spectroscopy shows that significant dissociation of this cation occurred on the time-scale of the electrochemical measurements. Much larger currents were also found for CMEs containing cis- or trans-[Ru(bpy)2(H2O)2]2+, and these are attributed to the greater mobility of adsorbed bis-bipyridyl cations.