The influence of oxygen on the catalytic properties of tungsten carbide has been studied for the reforming reaction of 2-methylpentane in a great excess of hydrogen (P-H2/P-HC = 150). The oxygen surface concentration has been measured by the ratio of the signal area of the O-1s and the W-4f levels as determined by X-ray photoelectron spectroscopy. Two kinds of processes are observed : cracking and isomerization reactions. The introduction of oxygen at T = 350 degrees C leads to a deep decrease of the cracking reaction kinetics; about two orders of magnitude. The tungsten carbide, strongly selective for cracking reactions in absence of oxygen, becomes very selective for isomer production in the presence of oxygen.