This paper describes in close molecular detail the complex reactions which occur when a variety of hydrocarbons are adsorbed on fresh or oxygen-modified bulk tungsten carbide catalysts. On fresh tungsten carbide without oxygen, cracking reactions are the only reaction processes occurring; the results have been interpreted with a kinetic treatment which assumes successive carbon-carbon bond ruptures. The conditions under which the bulk tungsten carbide powders are treated with oxygen have a determinantal influence on their catalytic properties. If air is introduced at moderate temperature (350 degrees C), an acidic character of the catalyst is observed which leads to a bifunctional mechanism. Cracking reactions are the most important processes, and corresponding mechanisms have been interpreted using the presence of OH surface groups leading to alkoxy species as intermediates. If air is introduced in flowing hydrogen at high temperature (700 degrees C), the metallic character of the material is observed, leading to cracking and isomerization reactions explained via classical metallacyclobutane and sigma-alkyl species, respectively.