The highly cationic nanoparticle [Au-225(TEA-thiolate(+))(22)(SC(6)Fc)(9)] adsorbs so strongly on Pt electrodes from CH3CN/Bu4NClO4 electrolyte solutions that films comprised of 1-2 monolayers of nanoparticles can be transferred to nanoparticle-free electrolyte solutions without desorption and ferrocene voltammetry stably observed (TEA-thiolate(+) = -S(CH2)(11)N(CH2CH3)(3)(+), SC6Fc = S(CH2)(6)-ferrocene, Fc = ferrocene). The Fc(+/0) redox couple's voltammetry is used to detect the adsorption The apparent formal potential (E-APP(o)') of the Fc(+/0) couple depends on the electrolyte-its anion, cation, and concentration-in the contacting nanoparticle-free solution A 10-fold change in electrolyte concentration shifts the Fc(+/0) E-APP(o)' by 48-67 mV, depending on the electrolyte The dependency is interpreted to reflect the energetics of transfer of charge-compensating anions from the electrolyte solution to the monolayer nanoparticle "phase", promoted by the formation of Fc(-) sites in the nanoparticle film This interpretation is supported by electrochemical quartz crystal microbalance results Some further aspects of the results suggest adsorption of electrolyte cations at the nanoparticle film/electrolyte solution interface The interface mimics a liquid/liquid interface between immiscible electrolyte solutions, in which the ion transfer approaches permselective behavior The experimental results show that even 1-2 monolayers of highly ionic nanoparticles can behave as a polyelectrolyte "phase".