The macroscopic Soret coefficient S-T was measured for three Porous media, namely mica, glass powder, and natural compact clay. At a mean temperature of T = 25 degrees C and with NaCl, S-T for mica and glass powder was found to be equal to (3.1 +/- 0.7) x 10(-3) K-1 and close to values for a free medium in agreement with theoretical predictions which are obtained under the assumption that the pressure gradient and the electric field are negligible on the pore scale. The main result is that for clay S-r was found five times larger, presumably because of extra couplings with electrical phenomena. This latter measurement was confirmed by an independent technique based on the membrane potential. (c) 2006 Elsevier Inc. All rights reserved.