Backdraft is a special fire phenomenon in a limited-ventilation building. It can develop from fires of either ordinary combustibles or ignitable liquids that become oxygen starved yet continue to generate a fuel-rich environment. If abundant fresh air is abruptly supplied by opening a door or breaking a window, the hot gas in this vitiated building will rapidly combust and a fire ball and a blast wave will take place. Backdraft has the characteristics of abrupt occurrence and powerful destruction, which may cause the death of people in situ and the collapse of the building. In this paper, the results of a reduced-scale experimental test series are presented. This reduced-scale compartment (length x width x height with 1.2 m x 0.6 m x 0.6 m) was fitted with one opening (width x height with 0.2 m x 0.6 m in the middle of the end wall). The experimental results show that the key parameter determining the occurrence of backdraft is the mass fraction of unburned fuel (i.e. unburned methane in this study). When the mass fraction of unburned fuel in compartment exceeds a critical value, i.e. 9.8% backdraft will take place. (C) 2003 Elsevier Science Ltd. All rights reserved.