Water‐sensitivity tests have been performed on samples of the Triassic Hopeman Sandstone from Clashach Quarry near Elgin in Scotland. Despite the apparent lack of clay minerals in the samples, the permeability of core plugs to NaCl and KCl brines decreases with progressive volume of efflunent, and drops dramatically when distilled water is flushed through. The permeability to CalCl brines remains constant, however, emphasising the stabilsing effect of polyvalent cations. These results are explained by cation exchange, expansion of electric double layers and the consequent migration of fine