The ageing under nitrogen flow at 250-450degreesC of coke formed during different model molecules (m-xylene, n-heptane, propene) over a USHY zeolite is discussed. This ageing treatment causes a decrease in the zeolite coke content (essentially for low reaction temperature) and an increase in the size and aromaticity of coke molecules. Pyridine adsorption followed by IR spectroscopy, carried out on the coked samples before and after ageing, shows that ageing causes a significant decrease in the concentrations of protonic and Lewis acid sites able to retain pyridine adsorbed at 150degreesC. The deactivating effect of coke is also more pronounced after than before ageing. A change in the mode of deactivation from site poisoning to pore blockage is proposed to explain these observations. The same trends were observed for ageing a commercial coked catalyst, sampled from the fluid catalytic cracking (FCC) unit.