Durability of the Ru/CeO₂/γ-Al₂O₃ catalyst for steam reforming of dodecane was investigated using a packed bed type tubular reactor with multiple gas sampling ports along its length. The catalyst, before and after the durability test, was characterized by BET and measurement of carbon deposition. The Ru/CeO₂/γ-Al₂O₃ catalyst showed nearly complete conversion of dodecane to H₂ enriched gas and stable performance for as long as 5000 h of time-on-stream. The technique using multiple gas sampling port allowed evaluation of the decay of the catalyst performance. The catalyst started to deactivate from the inlet of the catalyst bed, where severe carbon deposition was observed. Assuming that the number of active catalyst sites was decreased by carbon deposition, kinetic analysis was performed to evaluate the decay of catalyst activity. Steam reforming of dodecane may proceed through the formation of active hydrocarbon species bound to the active catalyst sites. At lower temperatures, the activated hydrocarbons bound to the active sites become less reactive and tend to form deposited carbon. As temperature increases, the reaction between the active hydrocarbon species and steam proceeds predominantly over carbon formation.