This paper reports the results of a numerical study on the transient removal of a contaminant from a two-dimensional enclosure with one inlet and one outlet. The influence of buoyancy forces due to thermal and concentration gradients, contaminant diffusivity, inlet velocity and outlet disposition over the cleaning-time are studied. The governing equations of the laminar flow (continuity, momentum, energy and contaminant concentration) are solved by means of the SIMPLEC algorithm. Several simulations of the same test case have been made, using two numerical schemes : PLDS and SMART, in order to determinate which one provides better performances. For isothermal situations, the time required to remove the contaminant is studied parametrically as a function of the outlet position and the governing dimensionless numbers (Reynolds number, Schmidt number and solutal Rayleigh number), while the effect of horizontal temperature differences is studied in several situations in which it plays an important role. Buoyancy forces are found to have a strong influence over the flows and, in consequence, over the cleaning times.