We present an experimental investigation of Rayleigh-Bénard convection in binary-gas mixtures. In order to interpret the results quantitatively, we determined the necessary thermodynamic and transport properties for six mixtures ~He-CO2, He-SF6, He-Xe, Ne-Ar, Ar-CO2, and H2-Xe! by a combination of data from the literature, molecular-theory calculations, and thermal-conductivity measurements. Al...