This paper addresses the potential augmentation of natural convection heat transfer in Rayleigh-Benard enclosures when filled with a certain type of binary gas mixture. To form the binary gas mixtures, helium (He) is the primary gas and the secondary gases are nitrogen (N-2), Oxygen (O-2), carbon dioxide (CO2) and methane (CH4). Each of the thermophysical properties participating in the binary gas mixtures viscosity eta(m), thermal conductivity lambda(m), density rho(m), and heat capacity at constant pressure C-p,C-m depends on the molar gas composition, temperature and pressure. Results are presented in terms of the maximum allied heat transfer coefficient h(m,max)/B at the optimal mote gas composition w(opt), in the w-domain [0, 1] for the entire range of laminar and turbulent conditions. In the conduction regime, He provides the best heat transfer regardless of temperature. In the convection regime at 300 K a He-CO2 mixture usually provides the maximum heat transfer, whereas at 1000 K pure methane CH4 is the optimum. In addition, a detailed thermo-fluidic structure of the thermal convection patterns in the Rayleigh-Benard enclosure was analyzed by performing 2-D numerical simulations. (c) 2005 Elsevier Ltd. All rights reserved.