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Studies have been made of the pressure‐induced flow of the chloride solutions of various monovalent, divalent and trivalent cations, at various concentrations, through compressed Wyoming bentonite and Willalooka illite cores.The majority of the measurements were carried out on the Willalooka illite core and its permeability has been shown to depend markedly on the nature and concentration of the cations present, with three‐ to fourfold variations being observed in some instances. At the lower electrolyte concentrations permeability increases in the order Na < Ca < Ba < La < Cs at a given concentration. In addition the permeability increases linearly with flow pressure to an extent ranging from some 15% for the Na systems to 97% for the 3.0M LaCl3 system over the pressure range examined.The contributions to the permeability changes arising from mechanical expansion of the flow cell, particle rearrangements and the electroviscous effect have been shown to be quite inadequate to account for the major changes observed. It is suggested that the most significant effects arise as a result of the influence of cationic size, charge, and concentration on the viscosity of the several layers of coerced water which form adjacent to the clay surfaces. Coercion of water may arise partly through its association with the exchangeable cations adsorbed on the surface or forming diffuse double layers, but mainly from the preferred or unique water structure developed in proximity to the clay surface. In particular increasing concentrations of Cs, Ca and La cations appear to cause a pronounced breakdown in water structure within the pores.