The forces between colloidal probes and several polymer films were measured by atomic force microscopy in the presence of a series of electrolyte solutions. For Nafion films using a negatively charged silica tip, a repulsive force was obtained at different concentrations of NaClO4. A similar result was obtained for an anion exchange membrane with a positively charged probe. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to calculate the surface potential and hence, the surface charge. The surface charge density (similar to0.3muC/cm(2)) was independent of electrolyte concentration. The slope for plot of potential drop vs ln[c(5)] was similar to0.020 V. A theoretical treatment based on GCS theory was employed to account for the above results. For a poly,(vinylferrocene), (PVF) film, potenial-dependent force curves were obtained, which were qualitatively different from that previously reported for an electronically conducting polymer film electrode.(1).