The transformation of o-xylene in low concentration (1700 ppmv) in air was investigated over a series of Pd/HFAU catalysts differing in their Pd content (from 0 to 0.8 wt.%) and their framework Si/Al ratio (4, 17 and 100). With all catalysts, o-xylene oxidation into CO2 and H2O is accompanied by the retention within the zeolite pores of heavy products of xylene transformation (＇coke＇). The relative significance of these reactions depends on the operating conditions (temperature, time-on-stream) and on the catalyst characteristics. The higher the reaction temperature, the faster the oxidation of xylene and the lower the coke formation. The time-on-stream has a positive effect on oxidation apparently related to the reducibility of Pd species and a negative one on coking due to the self deactivation of this reaction. For a given zeolite support, the rate of o-xylene oxidation increases with palladium content. On the other hand, the higher the Si/Al ratio of the zeolite, the lower the number of acid sites n(A), the slower the coke formation and the faster the o-xylene oxidation, this latter effect being related to a negative effect of acidity on Pd reducibility. (C) 2000 Elsevier Science B.V. All rights reserved.