Stearic acid/valinomycin mixed monolayers were transferred to solid substrates and were subjected to optical and electrochemical investigations to determine the role of stearic acid in assisting valinomycin to complex potassium ions. Direct-view dark-field optical observations of multilayer/water interfaces revealed relatively uniform surfaces for mixed films with large and small concentrations of valinomycin (10 and 70 mol %), while films containing 40 mol % valinomycin showed nonuniform clustered surfaces. Fourier transform attenuated total reflection infrared spectra showed the presence of complexed valinomycin molecules after samples were exposed to KCl solutions. Langmuir-Blodgett multilayer systems of valinomycin/stearate mixtures were exposed to water solutions of KCl and NaCl in a concentration range of 0.001-100 mM. The electric current generated by these multilayer systems in contact with the electrolyte solutions was dependent on the K+ concentration. In contrast, these systems were not sensitive to Na+.