A numerical study of combined heat and mass transfer by natural convection adjacent to vertical surfaces situated in fluid-saturated porous media is reported. Special attention is given to opposing buoyancy effects of the same order and unequal thermal and species diffusion coefficients. The numerical results support the validity of the boundary layer analysis for high Rayleigh number aiding flows and for opposing flows when one of the buoyant forces overpowers the other. For other cases, such as low Rayleigh number and opposing buoyant forces of the same order of magnitude, full solutions are needed. The structure of the flow, temperature and concentration fields are governed by complex interactions among the diffusion rates and the buoyancy ratio. Numerical results for velocity, temperature and concentration profiles are presented for an extensive range of parameters and the complex physical mechanisms underlying the flow and transport are clearly explained. Nusselt and Sherwood number data are given and the more subtle aspects of the problem are clarified.