The influence of the formation of oxygenated compounds on the adsorption properties of a Y zeolite were investigated. A procedure was developed which allows the zeolite to age under a flaw of toluene or p-xylene containing dissolved oxygen. The separation properties of the sieve were tested by a chromatographic method involving the separation of p-xylene and m-xylene. It was found that aging in the presence of oxygen leads to a dramatic decrease in both the capacity and selectivity of the sieve, while aging in a clean feed leads to stable separation properties. Together with the evolution of the sieve, impurities which are mainly benzaldehyde and water are produced in the feed. Heavy aromatic compounds also occur during aging. The oxidation of toluene to benzaldehyde and the subsequent formation of water may be responsible for the decreased performance of the sieve in the separation of p-xylene from C8 aromatics.