Experiments using water and brine were performed to simulate the mixing pattern above a slow steady circular release of light Fuel gas into a region of otherwise quiescent, heavier air. The experiments reveal the existence of a horizontal inward moving boundary- or mixing-layer flow above the release surface. Over the central region of the release area the mixing layer turns upward and feeds a vertical buoyant plume. Measured mixing layer thicknesses and solute salt concentrations were compared with predictions based on a concentration boundary-layer type model combined with a model for vertical flow into the plume. (C) 2001 Elsevier Science Ltd. All rights reserved.