The chemical analysis of the pore fluid of the Costa della Gaveta landslide (Italy), an earthflow in a marine origin clay formation, shows that it is an aqueous ion solution in which Na+ is the prevailing cation. The solution concentration varies noticeably with depth and, in some zones, the concentration in the landslide soil is much lower than that in the stable soil underneath. Laboratory experimental results relative to both reconstituted and undisturbed samples show that the mechanical behaviour of the considered soil is strongly affected by pore fluid composition in the range of concentration evaluated in the field. In particular, with ion concentration decreasing the residual friction angle decreases and viscous displacement rate under constant shear stresses increases. Pore solution concentration is thus a very important parameter to be determined and technical procedures which allow to evaluate it in large subsoil volumes should be developed. Pore solution concentration is correlated to the electrical resistivity of the soil. Thus, to evaluate the field pore solution concentration, it is suitable to calibrate and improve the electrical resistivity techniques, such as ERT (Electrical Resistivity Tomography).