This paper reports the results of a numerical study of the time-dependent removal of contaminant from a two-dimensional enclosure with one inlet and one outlet. The contaminant is generated beginning with the time t = 0 by a concentrated source located inside the enclosure. The contaminant is removed by the through flow established between the inlet and outlet ports. The flows studied cover the laminar and turbulent regimes represented by 30 ≤ Re ≤ 3000, where Re is the Reynolds number based on the inlet width and mean velocity. The effectiveness of the contaminant removal scheme is documented in terms of the removal efficiency ηr, the volume-averaged concentration of contaminant C̄, and the critical (clean up) time tc. The effects of Re, ventilation jet orientation and source location are reported. It is shown that the movement and distribution of contaminant is complex and depends strongly on the source location. It is also shown how the relative positioning of the ports and the source location influence the contaminant removal process. The optimal inlet/outlet configuration associated with each position of the concentrated source of contaminant is reported. Slower ventilation schemes can lead to lower contaminant levels when a short contaminant removal time is not a major requirement. © 1992.