Abstract Polycrystalline tungsten oxide (WO 3) pellets were prepared by conventional ceramic processing technology. The ac small-signal electrical data acquired in the frequency ( f) range 100 Hz≤ f≤1 MHz at temperature ( T) ranging the 31–100 °C revealed distinct semicircular relaxation in the impedance plane. This relaxation indicates device behavior originating from the grain boundaries. The lumped grain impedance associated with the device action remained too small to detect when the large resistance scale is realized. The semicircular relaxation is thermally activated indicating 0.58 eV as the activation energy for the relaxation time.