Abstract: For use as a replacement for existing gas to burners, oil having an API gravity of about 10-80 is preheated and mixed with about 0.1 to 1 pound of steam per pound of oil, at a pressure of about 30-150 pounds per square inch gauge. The resulting mixture is heated to a temperature above the condensation temperature of the mixture, the temperature being approximately in the range of 450.degree.-800.degree. F. The mixture has flow properties which are essentially equivalent to those of gas under the same conditions of temperature and pressure. The mixture is delivered to the nozzle of the gas burner and is burned therein.An apparatus is provided for burning either oil or gas, and includes a gas burner having nozzle means for mixing air and gas, a gas conduit connected thereto, and an oil supply also connected to the burner, the oil supply being fed from an oil vaporizing apparatus as just described above.