An experimental study of boil over phenomena in connection with crude oil and kerosene pool fires w a s carried out in the large scale t e s t facilities of the Fire Research Institute of Japan. Arabian light crude oil was burned in steel pans ranging from a circular one with a diameter of 0.3 m to a square one with a 2.7 m side. An oil layer of 10 mm to 100 mm was floating on water in the pans. The intensity of boil over w a s found to increase with increasing fuel thickness. An almost tenfold increase in the amount of unburned residue was found to occur as the layer thickness was increased from 40 mm to 50 mm. Results of measurements of liquid and gas temperatures, burning rates and radiative heat outputs are reported.