Experimental and numerical studies have been carried out on the combined phenomena of molecular diffusion and natural convection with a graphite oxidation reaction in a multicomponent gas system to investigate the process of air ingress into a reverse U-shaped tube consisting of one side heated and the other side cooled pipes. The range of the Grashof number based on the height of the tube was about 3.7 x 10(9) < Gr(L) < 4.7 x 10(11). One-dimensional basic equations for continuity, momentum conservation, energy conservation of the gas mixture, and mass conservation of the gas species are numerically solved to obtain concentration changes in the gas species and the onset time of the natural circulation of air. The experimental results showed that air entered the tube due to molecular diffusion and a very weak natural convection of the multicomponent gas mixed prior to the onset of the natural circulation of air. The calculated results are in good agreement with the experimental ones regarding the concentration changes in the gas species and the onset time of the natural circulation of air.