Extended catalytic tests show that coke deposition is responsible for the deactivation of activated carbon catalysts in the oxidative dehydrogenation of ethylbenzene (ODE). Temperature-programmed desorption (TPD), DRIFTS, textural and elemental analyses of used catalysts have shown that not only the great majority of the micropores become blocked, but also that the amounts of oxygen and hydrogen in the catalyst composition increase with time on stream, leading to a material increasingly more reactive towards oxidation. It was observed that after a few days on stream, the rate of carbon gasification became larger than the rate of coke deposition, leading to a decrease in catalyst weight. Working under milder conditions (lower temperature and oxygen partial pressure) delayed this effect. The increase in the concentration of surface groups with time does not result in a proportional increase in the activity of the catalysts, because the majority of the groups created are not active for the ODE reaction.