The present work investigates numerically the laminar natural convection heat and mass transfer in open vertical rectangular ducts with uniform wall temperature/uniform wall concentration (UWT/UWC) or uniform heat flux/uniform mass flux (UHF/UMF) boundary conditions. The vorticity-velocity formulation is applied to solve for the coupled momentum, energy and concentration equations. Results of dimensionless induced volume rate Q, average Nusselt number Nu and Sherwood number Sh are presented in terms of channel length L, buoyancy ratio N, Grashof number Gr, Schmidt number Sc and aspect ratio gamma. Analytical solutions for Q, Nu and Sh for the UWT/UWC case are derived under fully developed condition. In addition, the correlation equations of Q, Nu and Sh for both boundary conditions are also presented.