After an oxidative treatment, molybdenum carbide is an active and very selective catalyst for the isomerization of n-heptane. An important parameter in the choice of a catalyst for application in industry is its resistance to deactivation by coke formation, which can be caused by the presence of cyclic molecules. In this work, a study of the effect of methylcyclopentane (MCP) on the deactivation of the oxygen-modified molybdenum carbide catalyst has been performed. It has been found that the catalyst deactivates in the presence of pure MCP and that a lower activity for heptane isomerization is obtained after the reaction under MCP. TEM showed that coke formation is the cause of the deactivation. It has also been shown that increasing the total pressure when the MCP is reacted leads to a large decrease in the catalyst deactivation and that heptane isomerization can be carried out over this deactivated sample with only a small loss in activity. Further, the catalyst can be totally regenerated in air.