In order to gain reliable thermal diffusion measurements, which enable a microscopic interpretation of this effect, we have studied the hydrodynamical stability of a confined (LiCl, 9.7H2O) mixture submitted to both temperature and concentration gradients. A comparison with Rayleigh-Bénard experiments in binary mixtures has been undertaken that emphasizes the influence of the confinement. This study has permitted one to carry out interdiffusion and Soret coefficient measurements of the liquid, free from any convective disturbance. Opposite to initial predictions, the Soret coefficient appears to be largely negative and these results weaken some assumptions about the origin of the sign inversion of the effect, which is still questionable.