The phenomenon of natural convection caused by combined heat and mass transfer is studied numerically in a two-dimensional square cavity with insulated horizontal walls. The method is a combination of the Gauss-Seidel iterative technique and optimum under-relaxation. Results are obtained for steady-state flow with various: temperature and molar fraction boundary conditions for the vertical walls and various heat and mass Grashof number values, in the case when buoyancy forces due to simultaneous transfers are opposed.