This paper presents the results of an experimental and analytical study of the relative flammability hazard presented by the fuel tank headspace vapours of alcohol/gasoline blends. The aim was to determine if these blends pose a greater hazard in practice than conventional gasoline fuels.Three types of experiments were conducted. The first was a flammability test performed at high and low energy levels using the Bruceton method to assign a statistical probability of ignition at different temperatures. Vapour pressure measurement and gas chromatographic analysis allowed the experimental vapour phase flammability to be determined and a mathematical model to be validated. The model was used to examine the hazard parametrically for other conditions.Even with a high alcohol content, the characteristics of the vapour are strongly influenced by the more volatile gasoline fractions. The flammable range broadens and shifts to warmer temperatures as fill level decreases and alcohol content increases.