Carbonaceous deposits (coke) formed during the catalytic deep oxidation of benzene over copper and palladium zeolites Y have been analyzed by means of temperature programmed oxidation, FTIR spectroscopy and HRGC/MS techniques. The coke fractions insoluble in methylene chloride consist of polyaromatic compounds with different oxygen contents. The fact that the main constituent 1,4-naphthoquinone and five more identical components are found in the soluble coke fractions of PdY as well as CuY points to a similar reaction mechanism on both catalysts. Corresponding to the results a polymerization-oxidation mechanism is discussed. Starting from radical cations, polyaromatic compounds are formed which are decomposed via oxidic intermediates under formation of H2O and CO2. At elevated temperatures, the direct participation of oxygen from the gas phase seems probable. The lack of a soluble coke fraction and the comparatively low oxygen content in the insoluble coke fraction of HPdY supports the suggestion that the relatively low activity of this catalyst might be traced back to its restricted capability for coke decomposition.