The in situ deposition of coke by exposure to ethylene at 673 K on 2 wt.% Mo/Al2O3 reduced and sulfided catalysts has been studied by means of IR spectroscopy. The bands at about 1580 and 1470 cm(-1) indicate the presence of carboxylate carbon in the coke. The fact of CO adsorption suggests that Mo species contain mainly Mo5+ ions as well as coordinately unsaturated Al3+ ions. The hydroxy groups attached to these ions take part in coke formation.