A mixture of hydrocarbons, oxygen and steam is reacted over a fluidized bed, in a first reaction stage, at high temperature to produce, at hydrocarbon conversion levels ranging from about 85 percent to about 92 percent, a reaction product of hydrogen and carbon monoxide in admixture with unreacted hydrocarbons, carbon dioxide and steam. The level of hydrocarbon conversion is further increased by about 0.5 to 5 percent by contacting and endothermically reacting the reaction product mixture over a fixed bed of a high void fraction catalyst in a second reaction stage.