An experimental cell has been devised in order to measure the Soret coefficients in a compact clay, namely argilite, when a concentration difference of a binary mixture is applied simultaneously with a temperature difference. Temperature gradients have been imposed in the same direction as concentration gradients or in the opposite one. The sign of the Soret coefficients is related to the respective direction of these gradients. Generally, mass transfer was found to be enhanced by the Soret effect. Experimental values of the Soret coefficient are 41 given. (C) 2003 Elsevier Inc. All rights reserved.