Discs of composites of particular poly(aniline hydrochloride) or poly(aniline hydrotetrafluoroborate) and silver in a poly(ethylene oxide)-AgCF3SO3 polymer electrolyte were characterized using microwaves over the frequency range 0.5-3.0 GHz in a coaxial line. The materials demonstrated a rapid and reversible change in their microwave reflectivity when a de potential of 5 volts was applied across them. A coaxial line test set was used to observe changes of 15-20 dB in microwave reflectivities and displacements of ca 0.25 GHz in the resonance condition. The observations are tentatively discussed in terms of reversible changes in conductivities of the particulate components and the capacitance within ＇microcells＇ of polyaniline/polymer electrolyte/silver.