A quartz crystal microbalance (QCM) has been applied to study the adsorption of [Ru(bpy)(3)](2+) (bpy = 2,2＇-bipyridyl) and [Ru(NH3)(6)](3+) by a clay-modified electrode. An electrode was prepared by depositing an aqueous suspension of synthetic sodium saponite onto a gold-coated quartz crystal and drying it under air. After the electrode was swelled for 20 to 120 h in an aqueous 0.01 M Na2SO4 solution, a metal complex was injected into the electrolyte solution. The resistance of the electrode did not change during the adsorption process so that the mass change of the electrode was monitored in terms of the change of resonance frequency. In case of [Ru(bpy)(3)](2+), the saturated adsorption of racemic [Ru(bpy)(3)](2+) caused the mass increase of 0.49 +/- 0.03 g per 1 g of clay, while that of enantiomeric [Ru(bpy)(3)](2+) caused the mass increase of 0.29 +/- 0.02 g per 1 g of clay. On the basis of these values, it was concluded that racemic [Ru(bpy)(3)](2+) was exchanged with hydrated sodium ion ([Na(H2O)(4)](+)) in a clay film as a monovalent ion-paired species ([Ru(bpy)(3)](2+). 1/2SO(4)(2-)), while enantiomeric [Ru(bpy)(3)](2+) was exchanged with [Na(H2O)(4)](+) partly as a divalent ion and partly as a monovalent ion-paired species. Under the similar conditions, the saturated adsorption of [Ru(NH3)(6)](3+) caused the mass increase of 0.17 +/- 0.01 g per 1 g clay. The results indicated that [Ru(NH3)(6)](3+) was replaced with [Na(H2O)(4)](+) in a clay film as a monovalent ion-paired species ([Ru(NH3)(6)](3+) . SO42-). These results were compared with the adsorption experiments of the same complexes by colloidally dispersed sodium saponite.