The catalytic behaviours of bulk tungsten carbides differently modified by air treatments have been studied in hydrogenation (or deuteration) and double-bond isomerization of but-1-ene, in hydrogen/deuterium exchange of methylcyclopentane, as well as in hydrogen dissociation via H-2/D-2 exchange. Two kinds of catalyst samples have been studied. The first sample has been treated by air at moderate temperature (350 degrees C) and the second at high temperature (700 degrees C). It was found that, for but-1-ene reaction at temperatures varying between 30 and 100 degrees C, but-2-enes are the main products formed, and only a small amount of hydrogenated products was detected for both catalysts. Two possible double-bond migration mechanisms by an intramolecular hydrogen migration, either with pi-olefinic, pi-allylic and metallacyclobutane or with pi-olefinic and pi-allylic species interconversions are deduced; these mechanisms occur without the participation of dissociated hydrogen or deuterium atoms. However, it has been demonstrated that the hydrogen dissociation temperature is a function of the air treatment of the bulk tungsten carbide, even if this property has no effect on the hydrogenation of but-1-ene to n-butane. Thus, in order to see the influence of oxygen modification, a more detailed study has been carried out. Starting from a bulk tungsten carbide without oxygen, several controlled oxygen additions have been made, leading to four different samples. The HD formation has been analyzed for each sample as a function of temperature.