We address the problem of thermal diffusion in binary fluid mixtures, lying within a porous medium and subjected to a horizontal thermal gradient. Preliminary numerical results indicate that multiple convection-roll flow patterns can develop depending on the Soret number value, for counteracting thermal and solutal buoyancy forces. Investigating solute behaviour, the influence of these forces on solute transport is described in detail. Furthermore, it is shown that the theory represents well the solute behaviour when solutal buoyancy force is negligible. Finally, reproducing thermogravitational experiments by numerical means, dispersion is exhibited as a possible cause of the observed discrepancy between numerical and experimental results. The influence of dispersion on diffusive and thermal diffusive solute transports can be strong. (C) 2001 Elsevier Science Ltd. All rights reserved.