Transient laminar mixed convection in an asymmetrically and differentially heated vertical channel of finite length subject to an opposing buoyancy is investigated by solving the unsteady two-dimensional Navier-Stokes and energy equations. Results illustrate the effects of buoyancy strength or Richardson number Ri = Gr/Re-2 and Reynolds number Re on the overall flow structure and the nondimensional heat flux (Nusselt number) from the heated surface. Final steady or oscillatory flow response is obtained depending on the value of the Reynolds and Richardson numbers. The critical value of the buoyancy strength between the two regimes strongly depends on the value of the Reynolds number. (C) 2008 Elsevier Ltd. All rights reserved.