Various Cu/HZSM5-zeolites were prepared and their catalytic properties were investigated by productanalysis via GC/MS chromatography in order to trace down the mechanism of the gas phase one-stepoxidation of benzene to phenol with molecular oxygen. Comparison of Cu free and Cu containing zeolitesshowed that the activation of O2takes place at copper centers of the zeolite and high copper loadings leadto high yields of deep oxidation products (CO, CO2). No phenol was formed in the absence of Brønsted acidsites, i.e. on Cu/KZSM5, revealing the bifunctionality of the Cu/HZSM5 zeolite. The yields of the variousoxidation products and thus the selectivity toward phenol can be influenced by variation of the relativeO2concentration in the reaction mixture, indicating the possibility of a stoichiometric use of O2. The roleof the superoxide radical ion O2•−as a reactive intermediate is discussed and a radical ionic reactionmechanism is suggested.