This paper deals with streaming current generated by water flow through 4 geological porous media. These experimental measurements and a theoretical model allow us to determine the space charge density rho (w) at the wall in these 4 cases and to compare these values to previous ones obtained with glass capillaries and glass porous samples. The results show that rho (w) is effectively the intrinsic parameter of the solid-liquid interface because it is not a function of the geometry of the fluid flow paths but it only depends on the physico-chemical properties of the solid and the liquid. (C) 2001 The Institute of Physics. Published by Elsevier Science Ltd. All rights reserved.