Using C-13 solid-state nuclear magnetic resonance, the carbonylation of dimethyl ether (DME) with carbon monoxide has been studied on solid Rh/Cs2HPW12O40 in the presence of a methyl iodide promoter. The observed decrease in the reaction temperature in comparison with halide-free systems is caused by a change of the reaction mechanism. At first, the activation of the promoter and CO on Rh/Cs2HPW12O40 produces methyl rhodium carbonyl species. Carbon monoxide is then embedded into the Rh-CH3 bond to afford Rh acetyl. Rapid migration of the acetyl group from the Rh center to a Bronsted acid site of Rh/Cs2HPW12O40 gives rise to an acetate group attached to the Keggin unit (Keggin acetate). Bronsted acid sites provide dimethyl ether activation with the formation of a surface methoxy group. The latter, through reaction with hydrogen iodide, which is produced at the stage of Rh acetyl-to-Keggin acetate transformation, restores methyl iodide for subsequent carbonylation stages. Methyl acetate formation from a Keggin acetate and DME closes a catalytic cycle. (C) 2012 Elsevier Inc. All rights reserved.