The effect of isotopic exchange (D vs H) on the bulk electric conductivity and the static dielectric constant of acid polyhydrates, having a clathrate structure, were determined between 10 and 228 K (melting point). Protonated and deuterated acids show virtually identical Arrhenius plots of the conductivity above 120 K. Below that temperature they differ, the deuterated compound exhibits a smaller conductivity and a smaller dielectric constant as compared with the protonated form. The hydrogen and the deuterium evolution reactions were investigated at the interface Ag-solid 5.5 hydrated perchloric acid between 140 and 228 K. The gas evolution was found to depend strongly on temperature and isotope (D vs H). At the solid-solid interface the hydrogen evolution occurs faster than the deuterium evolution at the same overpotential. A temperature dependence of the charge transfer coefficient is observed with n alpha decreasing with decreasing temperature.