We have investigated the stability of an horizontal liquid layer heated from below, in the Rayleigh-Benard configuration, on a water-isopropanol-ethanol mixture. Neglecting the cross-diffusional effects, we have developed a linear stability analysis of the rest state and compared it with experiments. We did not find sufficient agreement. However, incorporating the cross-diffusion in the theory, it is possible to recover the critical parameters for the onset of convection.