Abstract: Numerical analyses were made for combined heat and mass transfer by natural convection on a vertical surface. Both boundary layer equations and full equations of momentum,energy and mass were solved by using a control-volume formulation, and the results were compared in detail for velocity, temperature, concentration, Nusselt and Sherwood number profiles. The discrepancies of boundary layer analysis from full equations under different values of buoyancy ratio B were systematically investigated for Pr/Sc>1,Pr/Sc<1 and Pr/Sc=1. The value of buoyancy ratio B_min predicted by boundary layer equations at the minimum heat and mass transfer rates was consistent with that of full equations solutions. The boundary layer analysis did not predict the velocity, temperature and concentration profiles accurately at Pr/Sc≠1 in the region near B_min. The maximum deviations of Nusselt and Sherwood numbers occurred near B=-1, and they were 40 percent and 21 percent respectively for Pr/Sc=3.18, and 40 percent and 35 percent respectively for Pr/Sc=0.14. The boundary layer equations might predict the heat and mass transfer rates rather accurately except the region near B_min, however, the deviation tends to increase with increasing ｜B｜.