A numerical study was made to investigate the laminar mixed convection heat and mass transfer in inclined rectangular ducts. Using the vorticity-velocity method, three-dimensional Navier-Stokes equations, the energy and concentration equations were solved simultaneously. Six independent parameters are introduced : Prandtl number Pr, mixed convection parameter OMEGA, modified Rayleigh number Ra*, buoyancy ratio N, aspect ratio gamma and Schmidt number Sc. The numerical results, including the local friction factor ratio, local Nusselt and Sherwood numbers, are presented for species diffusion of interest in air (Pr = 0.7) over a Schmidt number range of Sc = 0.2-2.0. In the study, the mixed convection parameter OMEGA ranges from 0 to 200, while the modified Rayleigh numbers Ra* are varied from 0 to 10(5) for the aspect ratios gamma = 0.5, 1, and 2.0 with buoyancy ratios N ranging from -0.5 to 2.0. The results show that the buoyancy effects have a significant influence on the characteristics of fluid flow, heat and mass transfer. Additionally, the fRe/(fRe)0, Nu and Sh are enhanced as the buoyancy force from species diffusion assists the thermal buoyancy force.