Steady-state, transient, and isotopic-exchange studies of dimethyl ether (DME) carbonylation, combined with adsorption and desorption studies of probe molecules and infrared (IR) spectroscopy, were used to identify methyl and acetyl groups as surface intermediates within specific elementary steps involved in the synthesis of methyl acetate from DME-CO mixtures with > 99% selectivity on H-zeolites. Carbonylation rates increased linearly with CO pressures but did not depend on DME pressures, suggesting that the addition of CO to CH3 groups present at saturation coverage controls catalytic carbonylation rates. These reactions lead to acetyl groups that subsequently react with DME to form methyl acetate (423-463 K; > 99% selectivity) and regenerate methyl intermediates, consistent with kinetic studies of CO reactions with CH3 groups previously formed from DME and with kinetic and IR studies of DME reactions with acetyl groups formed by stoichiometric reactions of acetic anhydride. These studies show that CO reacts with DME-derived intermediates bound on zeolitic Al sites from the gas phase or via weakly held CO species adsorbed non-competitively with CH3 groups. These reactions, in contrast with similar reactions of methanol, occur under anhydrous conditions and avoid the formation of water, which strongly inhibits carbonylation reactions. (c) 2006 Elsevier Inc. All rights reserved.