The flow field of a slot jet at low Reynolds numbers (Re = 39.4 and 78.8) was examined numerically and experimentally. The slot exit velocities are small, so the gravitational force has to be taken into consideration in numerical calculation. Experimentally, the fluid containing oxygen and nitrogen was issued from a slot jet into the quiescent atmosphere. The concentration field was measured by using a gas chromatograph and a gas-extracting quartz probe. Furthermore, smoke particles were used to visualize the flow field. Due to the interaction among momentum transfer, mass transfer and gravitational force, the flow and concentration fields at low Reynolds numbers with considerable mass transfer are found very different from those at high Reynolds numbers. The flow field is greatly influenced by the gravitational force due to concentration-induced density change; and the flow field in turn affects the concentration distribution. For an upward jet of density higher than air, at Re = 39.4, one vortex in the far region exists. However, at Re = 78.8, two vortices appear; one attached at the slot exit and one in the far region. The calculated concentration field is in fairly good agreement with the experimental result. (C) 2003 Elsevier Ltd.. All rights reserved.