This paper reports the results of a fundamental study of the transient removal of a contaminant from a two-dimensional enclosure with one inlet and one outlet. The evolution of the flow and concentration fields are simulated numerically using Jones and Launder's low Reynolds number k-ε model. The Reynolds number is varied over the range 5-5000, where Re is based on the jet inlet width. The effectiveness of this forced convection mass transfer process is documented quantitatively in terms of a ventilation efficiency and a critical concentration decay time. The relationship between these quantitative performance parameters, the Reynolds number and the ventilation jet orientation, is reported. It is shown that significant gains in ventilation efficiency (or a shortening of the critical concentration decay time) can be made by properly orienting and positioning the inlet and outlet ports relative to each other and to the enclosure. The numerical ventilation efficiency results are summarized by a compact analytical expression based on a theoretical two-zone model of the enclosure flow. © 1991.