A numerical investigation of two-dimensional interaction of two non-isothermal opposing jets/streams of different fluids (same phase, miscible) in an adiabatic channel is carried out in the mixed convection regime. The thermal buoyancy as well as intrinsic buoyancy (inherent differences in the densities of the two fluid streams) has been considered through the Boussinesq approximation. The two buoyancy forces give rise to thermal and intrinsic Richardson numbers, Ri(T) and Ri(C), respectively as the important dimensionless numbers governing the problem. Two configurations, namely; (i) the upper stream of heavier fluid at lower temperature and lower stream of lighter fluid at higher temperature (aiding buoyancy forces), and (ii) the upper stream of lighter fluid at lower temperature and lower stream of heavier fluid at higher temperature (opposing buoyancy forces), are considered. For both the scenarios, the simulations have been carried out for various combinations of RiT and Ri(C). The numerical experiments reveal the existence of three flow modes depending upon the flow configuration and the magnitude of the buoyancy forces. These are (i) steady asymmetric flow (ii) steady symmetric flow and, (iii) the unsteady, periodic, non-symmetric flow with formation of standing waves and vortex-shedding in the channel. At sufficiently large buoyancy levels, the unsteady periodic flow mode with standing waves is observed for the aiding configuration only. The mixing process is quantitatively monitored through a scalar mixing index that represents the mean square deviation of fluid temperature/mixture concentration from the bulk values at a given channel section. It is shown that in the steady symmetric flow mode, the buoyancy has a very slight effect on the mixing characteristics. However, the mixing is significantly enhanced for the unsteady, periodic standing wave mode. (C) 2010 Elsevier Ltd. All rights reserved.