Remarkably high mixed proton and electron conduction arising from oxidized single-wall carbon nanotubes at room temperature is demonstrated. The respective proton and electronic conductivities are 1.40 and 8.0 × 10⁻² S cm⁻¹ in the in-plane direction, and 3.1 × 10⁻² and 1.1 × 10⁻³ S cm⁻¹ in the out-of-plane direction, indicating their potential in a wide range of solid electrolyte membranes.