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An experimental study of the development and mitigation of backdrafts shows that the key parameter for backdraft development is the fuel mass fraction. The results reveal that the critical fuel mass fraction, Yf, required for the development of diesel fuel backdrafts is 0.16 for fully vitiated conditions. Analysis of the data in conjunction with results in the literature has also demonstrated that standard flammability diagrams can be used to predict bounding limits and trends on Yf with respect to other key variables, such as oxygen concentration. The injection of water spray was shown to be an effective mitigating tactic that was able to completely suppress backdrafts. The analysis reveals that backdraft suppression occurred primarily by means of diluting the atmosphere and reducing the fuel mass fraction, consistent with the critical Yf criterion, rather than by a thermal mechanism of cooling.