In order to clarify the mixing that occurs in a gravity current which precedes a backdraft, a two-dimensional simulation, a series of salt water experiments, and backdraft experiments were performed. A compartment in a ratio width/height/length of 1/1/2 is used in the experiments and computations. Two di!erent openings were used in the salt water experiments and numerical computations: a fully open end wall and a h /3 horizontal slot centred in an end wall, where h is the compartment height. For the backdraft experiments only the h /3 horizontal slot was used. The visual observations from the salt water experiments compare well with the numerical simulation. Both show a small mixed region at the gravity current shear interface for the fully open wall and mixing throughout the entire current for the h /3 horizontal slot. Quantitative comparisons are made in terms of the normalised density di!erences, " ( ! )/ , where is the higher density and is the lower density. The transit time results, i.e., time between opening the compartment and the time the gravity current reaches the wall opposite the opening, for the computations compared favourably with the transit times from the salt water experiments, over the range 0.003( (0.100. Velocity measurements from the opening of the backdraft compartment, prior to ignition, also are favourable with the numerical simulation.