The effect of doping potassium on the A-site of BaZrO3 for application as a high-temperature proton conductor was investigated. The synthesis of K-doped BaZrO3 and its characterization by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis is described. Potassium-doped material was stable at high water-vapor pressures (similar to 180 bars) and up to 500 degrees C, the maximum temperature of the autoclave treatment. Four-probe dc conductivity was measured on dense and porous (44% porosity) samples in varying partial pressures of oxygen, p(O2), over a temperature range from 500 to 700 degrees C. Conductivity was measured as a function of atmosphere and temperature on porous samples to ensure attainment of equilibrium conditions in a reasonable time period. The dependence of total conductivity in a dry atmosphere on oxygen partial pressure could be described by the equation sigma(t) = sigma(i) + sigma(o)(h)p(O2)(1/4). The experimental data were fitted to this equation with good accuracy, from which the electron-hole and oxygen-ion conductivities were determined. The activation energies for oxygen-ion and electron-hole conduction were estimated to be 0.85 and 1.14 eV, respectively. Conductivity measurements were also performed in H2O-containing atmospheres as a function of water-vapor pressure, p(H2O). A significant increase in total ionic conductivity was observed in humid atmospheres, which is attributed to high protonic contribution. Samples doped with potassium exhibit significantly higher ionic conductivity in humid atmosphere as compared to samples doped with yttrium. The enhanced ionic conductivity, which is mainly protonic, is attributed to the higher basicity of potassium. At high p(O2) and low p(H2O), potassium-doped BaZrO3 exhibits predominantly electron-hole conduction, while at low p(O2) and high p(H2O) it exhibits predominantly protonic conduction. (c) 2006 The Electrochemical Society.