The kinetics of the partial oxidation of isobutane with molecular oxygen on Rh(111) single-crystal surfaces were studied by using a collimated molecular beam under ultrahigh vacuum conditions. Both hydrogen and water were shown to form as primary products, not after secondary reforming or water-gas shift steps as it has been suggested in the past. The production of carbon monoxide (but not of carbon dioxide). was also detected. Water production reaches its steady-state rate in a slower fashion than the rest of the products, presumably because of the kinetics of formation and consumption of the hydroxo surface intermediate involved.