In situ complex impedance spectroscopy of H2O-bearing wadsleyite was performed in a multianvil apparatus at 14 GPa at temperatures up to 950 °C in order to determine electrical conductivity. With increasing H2O content in wadsleyite the electrical conductivity increases at a rate higher than observed in previous studies. The activation enthalpy in the temperature range studied where proton conduction dominates is low (0.66 eV) suggesting an inevitable crossover to small polaron conduction at moderately higher temperatures, depending on H2O concentration. Although the solubility of H2O in wadsleyite is significant for a silicate mineral (>3 wt%), the presence of more than trace dissolved H2O in wadsleyite is likely to result in a conductivity too high compared to recent estimates of transition-zone conductivity. The use of complex impedance spectroscopy shows that the frequency dependence of electrical properties is very different in the case of H2O-bearing silicate phases. At frequencies below 1000 Hz complex impedance spectra contain strong features which likely result from the sample–electrode interface such that including the low-frequency data would lead to artificially low conductivities.