The electric field dependence of the optical properties of a series of anion-conducting polymer electrolytes at an ITO-electrolyte interface was investigated. A change in reflectance and refractive index of polymer electrolytes [poly(ethyl methacrylate)](18)(Bu3SnX)(3)Bu4NX where X = Cl, Br, and SCN was observed. This was ascribed to anion accumulation/depletion in the interfacial region. Shorter response times were observed for electrolytes with higher conductivities, illustrating the interrelationship between these two phenomena.