Summary During the last years several flash over simulators have been built in Finland and Sweden for the use of fire brigades. The simulators are intended for training the extinguishing personnel. The simulator imitates the conditions in a typical room fire and it can be used, for instance to: give the trainees a practical experience of the flash over phase in a room fire and the endurance in such a room, teach extinguishing personnel to make observations about the development of the fire and to predict possible dangerous situations, teach the importance of appropriate and sufficient protective equipment, teach the extinguishing personnel correct and effective methods for confining and extinguishing room fires. A typical flash over simulator consists of two steel transportation containers, one of which is the actual fire compartment and the other an observation room for the trainees and the instructors. The walls and the ceiling of the fire compartment are lined with combustible materials, that during combustion will cause a burning gas layer in the ceiling of the simulator. From the observation room one can observe the development of the fire and practice extinguishing. Flash over simulators have until recently been designed and developed mainly by the fire brigades based on their own experiences and practical knowledge. Phenomena influencing its operation have not been systematically investigated, neither have physical parameters in this context been measured so the total understanding of the simulator has thus been very insufficient. The properties and operating conditions have been measured in this investigation through practical tests. The safe operating range of the simulator has been determined from the results and based on this, a proposal for a suitable instructor's guide has been drawn up. A miniature model of the flash over simulator has also been built, which has been used for producing a demonstration video tape on the gas fire and flash over phenomena.