Contact of a flammable liquid and water can cause hazardous boilover because of their different boiling points. The dependence of the initial boilover point of a flammable liquid on atmospheric pressure was studied at different geographic locations with different altitudes, i.e., at HeFei (alt., 0 m; P, 101 kPa) and on Lhasa plateau (alt., 3650 m; P, 64 kPa). The temperatures in the bulk and at the interface of the flammable liquid and water, flame temperature, thermal radiation, sound level, and mass loss at ambient temperature were experimentally measured using identical transparent quartz pans for each measurement. The experimental results showed that boilover was delayed or diminished at low pressure and disappeared completely at pressures lower than a certain critical value for this pan surface area. The pan size was shown not to affect the boilover onset or intensity.