The behaviour has been examined of two samples of Berea and Vosges sandstones submitted to the circulation of solutions of different natures, at different temperatures. Solutions of CaCl2 and NaCl at ionic strengths I = 0.01, I = 0.1 and I = 1, taken in the increasing and then decreasing directions, were heated to temperatures of 20, 30, 40, 50, 60, 70, 80 and 90 degrees C. These temperatures were taken in the increasing and then decreasing directions. For the two sandstones, each temperature variation in the percolating solution was accompanied by a sharp variation in permeability, in the opposite direction to the temperature variation. The permeability versus temperature variation describes hysteresis loops whose area is greater when hot than when cold. These curves are typical of two types of behaviour. The first is obtained with diluted solutions (I = 0.01 and I = 0.1), and is characterized by a greater permeability during the rising temperature cycle than during the decreasing temperature cycle. The second, obtained with concentrated solutions, is characterized by an inverse variation: the permeability is greater during the decreasing temperature cycle than during the rising cycle. These different behaviours are interpreted in terms of flocculation-deflocculation of the clays contained in the porous media.