The developing laminar mixed convection heat and mass transfer in inclined rectangular ducts has been studied numerically, with six independent parameters : Prandtl number Pr, mixed convection parameter Omega, modified Rayleigh number Ra*, buoyancy ratio N, Schmidt number Sc and aspect ratio gamma. Typical developments of velocity, temperature and concentration profiles are shown to include the limiting cases of horizontal and vertical rectangular duels. Local friction factor, Nusselt number and Sherwood number are presented for species diffusion of interest in air (Pr = 0.7) over a Schmidt number range of 0.2-2.0. The results show that the Nu(Sh) is characterized by a decay near the inlet due to the entrance effect; but the decay is attenuated by the onset of buoyancy-driven secondary flows. The Nu (Sh) falls asymptotically to the value of Graetz problem when the temperature (concentration) profile becomes fully developed. The fRe/(fRe)(0), Nu and Sh are enhanced as the buoyancy force from species diffusion assists the thermal buoyancy force.