The natural convection of air in a square cavity wit horizontal temperature and concentration gradients which give opposing buoyancy forces near the vertical walls was studied numerically. The basic equations sloved are the continuity equation, the Navier-Stokes equations, the energy equation and the concentration equation. Computations were carried ot for Prandtl number Pr=0.71, aspect ratio A=1, Rayleigh number Ra=10~8, buoyanc ratio(-solutal buoyancy/thermal buoyancy) N=2 and various Lewis numbers Le. For Le=0.5 a strong main flow was formed near the walls and then a stratified layer in both the temperature and the concentration fields was formed between two convection layers. Secondary samall rol cells in each cnvection layer migrated periodically. For Le=1 and 2 the flow gradually became steady. For Le=5 a complicated pattern of small roll cells appeared near the walls.
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