Design and construction features ore given for a 12-foot diameter spherical steel vessel which has been added to U. S. Bureau of Mines facilities at Bruceton, Pa., for use in combustion research. The vessel is constructed of A-212B Firebox quality steel which has high tensile strength and resistance to brittle fracture. The maximum design of the vessel is 300 p.s.i.g. It is equipped with both inner and outer doors which permit entry of personnel for installation of instrumentation, and make possible experiments at elevated, atmospheric, and reduced pressures. The relatively large size of the vessel makes it particularly suitable for ignition, flammability, and detonability studies with minimum wall effects. It can be used for experiments with solid explosives as well as vapors and gases.