A tapered element oscillating microbalance (TEOM) was used to measure the rates of adsorption and desorption of p-xylene from FCC catalyst at 473 K. The original aim of the work was to investigate whether coke in the catalyst, formed by an earlier reaction with isopropanol at 773 K, influenced the adsorption kinetics. It was actually found that p-xylene reacted in the coked catalyst, even though no reaction occurred in the fresh catalyst at this temperature. This is a rare direct observation of coke causing chemical reaction in a system in which no reaction occurs in the absence of coke. (C) 2004 Elsevier B.V. All rights reserved.