This paper presents a numerical study of mixed convection heat and mass transfer in horizontal rectangular ducts. The vorticity-velocity method with the Du Fort Frankel scheme is employed to solve the governing equations for the flow, heat and mass transfer. Variations in local friction factor ratio, Nusselt number and Sherwood number with different parameters are shown for species diffusion of interest in air (Pr = 0.7) over a Schmidt number range of Sc = 0.2-2.0. In this work, the Rayleigh numbers are varied from 0 to 10(5) for aspect ratios 0.5, 1 and 2 with buoyancy ratios ranging from -0.8 to 2.0. The results show that the distributions of local Nusselt (Sherwood) number are characterized by a decay near the inlet in which the forced-convection entrance effect dominates; but the decay is attenuated by the onset of buoyancy-driven secondary flow. After a local minimum being researched, maximum and minimum local Nusselt (Sherwood) numbers may exist for some cases. Finally, the Nu (Sh) falls asymptotically to the value of Graetz problem when the bulk temperature (concentration) approaches the wall temperature (concentration). Additionally, the f Re, Nu and Sh are found to increase and decrease as the buoyancy force from species diffusion assists and opposes, respectively, the thermal buoyancy force.