In this work, the hydration and protonic conductivity of H-montmorillonite were investigated. The hydration water state,, as characterized by DTA, TGA and XRD at different environmental relative humidities (RH). The results, showing good reproducibility and consistency, mark two distinguishable categories of hydration water: molecular H2O and H3O- species, those coordinated with the proton, The proportion of each was quantitatively analyzed. The electrical conductivity measured by impedance spectroscopy increases with RH front 10(-7) Omega(-1) cm(-1) at 0.2 RH up to 0.6 x 10(-2) Omega(-1) cm(-1) at 0.9 RH at 20degreesC. The mechanism of protonic conduction involved in H-montmorillonite is also discussed.