X-ray photoelectron spectroscopy (XPS) has been used to determine the state of the transition metal ions (V, Cr, Fe, W, and Mo) on impregnated and ion-exchanged H-ZSM-5 zeolites that are active for the catalytic conversion of methane to benzene at 750 degrees C. During an induction period, transition metal suboxides are formed on all of the impregnated catalysts except Mo/H-ZSM-5 for which Mo2C is formed. Ion-exchanged samples became more active after treatment with CO at 500 degrees C. For the Fe/H-ZSM-5 sample, this treatment resulted in the migration of the metal to the external surface of the zeolite and the formation of an iron oxide phase.