The waste gases obtained during the pyrolysis, in baffled metal tubes, of 70:30 propane-butane mixtures at temperatures between 800° and 850 °C., with the object of producing liquid aromatics, have been used to produce carbon black in an experimental channel plant. The hydrocarbon mixture if utilized directly to produce carbon black yielded up to 4.9 lb. per 1,000 cu. ft. When the mixture was subjected to pyrolysis about 33 lb. of liquids (23 lb. in the gasoline range) was produced, while the off-gas yielded 5.4 lb. of carbon black. Yields of carbon black up to 10 lb. per 1000 cu. ft. were obtained with lower yields of liquid. Experiments on synthetic gas mixtures showed that the olefines were largely responsible for the production of carbon black from the pyrolysis off-gas. A large number of tests on rubber indicated that the carbon reinforces rubber in a manner as good as or better than that of the commercial gas blacks at present used in the rubber industry.