The vaporization of a water phase during a tank fire in a mineral oil storage tank may cause extensive damage due to the spillage of burning material and the intensified heat radiation. This phenomenon is known as ＇boilover＇. The consequences of a boilover mainly depend on the heat transfer mechanisms in the oil and its ability to form a layer of uniform temperature and composition but increasing extent under the surface, i.e. the hot zone. The heat transfer mechanisms in different fuels were studied experimentally. Models were set up to describe them theoretically. The consequences of water vaporization for the different fuel types were quantified and counter-measures are proposed. The results are important for the safe layout of tank farms and appropriate fire-fighting measures.