This paper studies the effect of unequal walls temperatures for a two-dimensional, two-sided, lid-driven cavity via numerical simulations using a multi-domain, spectral element method. The flow is driven by steadily moving the two opposite walls vertically in opposite directions. Different temperatures are applied on the two moving walls and the effect of the resulting heat transfer on the two-dimensional flow is studied. An analysis of the flow evolution shows that, with increasing the temperature difference between the opposite moving walls, the steady state flow changes from a single-vortex to a two-vortex pattern. Energy balance studies are conducted at steady state and during flow evolution. The evolution and distribution of heat transfer at all walls and work done at the moving walls are studied. (c) 2007 Elsevier Ltd. All rights reserved.