The hydrogen (H2) electrode reactions were investigated in three kinds of imide-type ionic liquids having 1-n-butyl-1-methylpyrrolidinium (BMP+), trimethy-n-hexylammonium (TMHA+) and 1-ethyl-3-methylimidazolium (EMI+) as cations. The diffusion coefficient and solubility of H2 were determined by chronoamperometric technique using both macro- and ultramicrodisk electrodes. The diffusion coefficient of HTFSI was also examined by dissolving bis(trifluoromethylsulfonyl)imide acid (HTFSI) in the ionic liquids. The solubility of H2 was nearly constant in each system within the temperature range from 25 to 70°C The difference in the solubility of H 2 may be explained by the difference in the free volume estimated from the density and the molar volume of ions. The order of the diffusion coefficients of H2 and HTFSI in each system indicates that the viscosity is a dominant factor on the diffusion process of HTFSI, while the diffusion of H2 is not affected by the viscosity. copyright The Electrochemical Society.