Kinetic isotope effect and isotopic tracer/exchange methods were combined with in situ infrared spectroscopy and kinetic data to determine the mechanism of dimethyl ether (DME, CH3OCH3) oxidation to formaldehyde (HCHO) on MoOx/Al2O3. The reaction intermediates and elementary steps established a redox mechanism that led to kinetic rate equations that are consistent with observed dependencies of reactant pressures. Methoxide concentrations as detected by in situ infrared spectroscopy correlated directly with formation rates to establish their importance for the formation of HCHO and CH3OH. Reactant partial pressure studies showed that rates of HCHO and CH3OH formation are first-order in DME and zero-order in O-2 at low DME pressures. At high DME pressures, rates became independent of DME pressure and acquired positive-order O-2 dependencies. H-D kinetic isotope effects indicated that C-H bond activation is not involved in kinetically relevant steps and transient studies involving (CH3OCH3)-O-16-O-18,-(MOOx)-O-16/Al2O3 confirmed the kinetic relevance of DME dissociative adsorption, the step that precedes C-H bond activation. These studies also indicated that mechanisms for HCHO formation do not discriminate between methoxide species formed from DME oxygen and those formed from lattice oxygen. Transient studies with (CH3OCH3)-O-16-O-16(2)-O-18(2)-(MoOx)-O-16/Al2O3 did not lead to detectable O-16-O-18 levels, indicating that vacancy reoxidation is irreversible.