The unusually strong, irreversible adsorption of monolayer quantities of the P2Mo18O626- anion on glassy carbon, highly ordered pyrolytic graphite, indium tin oxide, and gold-coated quartz electrodes was examined by electrochemical, spectroelectrochemical, and quartz crystal microgravimetric techniques. A very simple method was developed to deposit multiple layers of composites consisting of alternating layers of the P2Mo18O626- anion and large, multiply-charged cations such as Os(bpy)(3)(2+). The resulting deposit is stable and exhibits electroactivity from both of the components that reflects the structure and ionic conductivity of the solid multilayer.