The convective stability range of nonreactive binary gas mixtures (Xe:He,Xe:Ar) in a closed vertical cylinder with conducting walls heated from below was investigated. The xenon mole fraction was varied fromXXe= 1 to 0.03 and from 1 to 0 for the helium and argon containing mixtures, respectively. Critical thermal Rayleigh numbersNRa1for the onset of convective flow were obtained from high resolution differential temperature measurements. Thermal diffusion ratios were calculated, based on Lennard–Jones potentials, over the whole composition range for both gas mixtures. Minima inNRa1occurred at the composition for which maximum relative mass separation of the components due to thermal diffusion is predicted. Specifically, the monocomponent valueNRa1≊200 for a cylinder with height‐to‐radius ratio of 6 was reduced to 25 atXXe≊0.03, and 165 atXXe≊0.3 for the Xe:He and Xe:Ar systems, respectively. Comparison with Rayleigh–Be´nard instability theory, including Soret and Dufour effects, shows that for (gas) mixtures, the lateral walls act convectively less stabilizing than for monocomponent systems. It is shown that Dufour contributions to convective stability in these gas mixtures are negligible.