X-ray diffraction, base exchange studies and core flood tests were made to explain the reduction in permeability of Berea sandstone which results from changing the salinity of flood water. It was shown that the reduction of permeability is related to the exchangeable cations on the clays and to particle morphology. In the Berea sandstone, the clays were found to contain primarily Ca++, Mg++ and K+, but very little Na+. At this exchange state, the clay crystals are held together tightly, cannot swell to large interlayer spacings and no damage occurred when distilled water was flowed through the cores. However, if NaCl solution is flowed through a Berea core, the exchange sites on the clays become filled predominantly with Na+. The particles can now expand by hydration of the sodium. This swelling increases the distance between clay crystals and weakens the bonds holding them together. When the clays are subsequently exposed to distilled water, the double layer is removed by moving pore water, leaving adsorbed sodium cations on the faces of the clay crystals. These cations now become more strongly hydrolyzed, further weakening the bond between particles so that they now separate under the shear stresses generated by flow. If the clays are first exposed to Ca++ or Al+++, the clays are converted into these exchange cations. The stronger coulombic attraction of the divalent or trivalent cations holds the clay crystals together, preventing swelling or dispersion during subsequent fresh water flow. Particle morphology plays an important role in the dispersion phenomena. X-ray diffraction work showed that many of the clay crystals in Berea sandstone occur as micro-aggregates. These micro-aggregates consist of clay crystals that have grown contemporaneously and which are so interpenetrated that they will not separate under moderate conditions. It was shown that an osmotic-type phenomena cannot account satisfactorily for the dispersion of the clays in Berea. Solutions with the same osmotic pressures as the NaCl solution used were found to cause no permeability damage when followed by distilled water flow.