A high-purity, ramsdellite-crystal type manganese dioxide (Koyanaka et al., 2005 [1]; Iikubo et al., 2010 [2]) was used for an electrolyte in a hydrogen gas sensor (Ueda et al., 2011[3]). In this report, the electrochemical properties of the hydrogen gas sensor using electrolytes made of different crystal types of manganese dioxides, such as the ramsdellite-crystal type, a beta-crystal type, and a lambda-crystal type were examined. The high-purity, ramsdellite-crystal type manganese dioxide showed the conductivity from 7.1 x 10(-5) S/cm (80 degrees C) to 1.7 x 10(-4) S/cm (25 degrees C) under 85% relative humidity condition. This conductivity was probably based on the proton conduction on the MnO2 particles. (C) 2012 Elsevier B.V. All rights reserved.