Smectites or swelling clays are lamellar minerals made by the stacking of two tetrahedral silica sheets with one octahedral layer inserted of brucite or alumina type. Conductivity measurements and cation diffusion coefficient determination on different smectites, with varying water content, show that the conductivity is mainly protonic and depends on the water content. Structure and micro dynamics of adsorbed water is studied by XRD, IR, QENS, NMR spectroscopies. The protonic conductivity (10(-4) S cm(-1)) exhibited by these minerals is associated with the proton diffusional motion (D = 6 x 10(-6) cm(2) s(-1)) : its value fits the Einstein relation well with the residence time tau(0) = 10(-10) s and the jump length 2.8 Angstrom deduced from high resolution QENS measurements.