The reactions of labelled 2- and 3-methylpentanes were carried out on three different oxygen modified bulk tungsten carbides varying by their oxygen treatment temperature. In all cases, isomerization took place ia bond shift mechanisms; no cyclic mechanism was involved in contrast to metals like Pt, Pd or Ir where such mechanism occurs. Alkoxy and σ-alkyl intermediates were put forward as adsorbed species responsible for isomerization for bulk tungsten carbides treated by O₂ at moderate temperature (350°C) over a short period of time (5 min) and at high temperature (700°C) for 4 h, respectively. These intermediate species are respectively correlated to an acidic and a metallic behavior of the catalytic surfaces. The increase of the O₂ treatment time at 350°C results in the presence of both kinds of these mechanisms for isomerization, a bifunctional behavior takes place where metallic and acidic characters are present. To explain the presence of terminal-¹³C on acidic catalysts, a “protonated cyclopropane alkoxy species” intermediate responsible for isomerization was proposed. The driving force for the formation of such species being the presence of stable alkoxy species, general reaction pathways are discussed for the different kinds of catalysts.