In situ IR spectroscopy was used to observe the intermediates formed on zeolites FAU and MFI during the synthesis of methyl methoxyacetate (MMAc) via carbonylation of dimethoxymethane (DMM) and the disproportionation of DMM to dimethyl ether (DME) and methyl formate (MF). Both reactions are initiated by the reaction of DMM with the Bronsted acid protons of the zeolite to form methanol and methoxymethoxy groups (MMZ). The latter species then undergoes one of two processes - carbonylation to form methoxyacetyl species, the precursors to MMAc, or reaction with DMM, resulting in DMM disproportionation. Surface intermediates for both DMM carbonylation and disproportionation respond to changes in reaction conditions in a manner consistent with observed steady-state kinetics. DMM carbonylation occurred more rapidly in the presence than absence of physisorbed DMM, a phenomenon attributed to solvation of the carbocationic transition state involved in the addition of CO to MMZ predicted by DFT calculations. The surface concentration of the methoxyacetyl species at steady state was 10 times smaller on FAU than on MFI, consistent with the higher rate of DMM carbonylation on FAU. Rate expressions for the formation of each product, based on the proposed mechanisms, in combination with a suitable set of rate coefficients, give a good description of the experimentally observed dependences of the rates of product formation on temperature and the feed partial pressures of CO and DMM. (C) 2010 Elsevier Inc. All rights reserved.