Dimethyl ether (DME) reacts to form formaldehyde with high selectivity at 500-600 K on MoOx-ZrO2 catalysts with a wide range of MoOx surface density (0.5-50.1 Mo/nm(2)) and local structure (monomers, oligomers, MoO3 crystallites, and ZrMo2O8). Reaction rates (per Mo-atom) increased markedly as MoOx surface density increased from 2.2 to 6.4 Mo/nm(2) and two-dimensional polymolybdates and MoO3 clusters became the prevalent active species. The rate of incipient stoichiometric reduction of MoOx-ZrO2 samples in H-2 also increased with increasing MoOx surface density, suggesting that catalytic turnovers involve redox cycles that become faster as the size and dimensionality of MoOx domains increase. DME reaction rates (per Mo-atom) decreased as MoOx surface densities increased above 6.4 Mo/nm(2), as MoO3 and ZrMo2O8 clusters with increasingly inaccessible MoOx species form. On MoOx and ZrMo2O8, areal reaction rates reach a constant value at MoOx surface densities above 10 Mo/nm(2), as the exposed surfaces become covered with the respective active species. ZrMo2O8 surfaces were more reducible in H-2 than MoOx surfaces and showed higher areal reaction rates. Reaction rates were nearly independent of O-2 pressure, but the reaction order in DME decreased from one at low pressures (< 40 kPa) to zero at higher pressures (> 60 kPa). DME reacts via primary pathways leading to HCHO, methyl formate, and COx, with rate constants k(1), k(2), and k(3), respectively, and via secondary HCHO conversion to methylformate (k(4)) and COx (k(5)). Primary HCHO selectivities (and k(1)/(k(2) + k(3)) ratios) increased with increasing MoOx surface density on MoOx-containing samples and reached values of 80-90% above 10 Mo/nm(2). Kinetic ratios relevant to secondary HCHO reactions (k(1)/[(k(4) + k(5))C-Ao]; C-Ao inlet DME concentration) also increased with increasing MoOx surface density to values of similar to0.1 and 0.8 on MoOx and ZrMo2O8 structures (at the constant inlet DME concentration C-Ao), respectively. Thus, increasing the coverage of ZrO2 surfaces with MoOx or ZrMo2O8 leads to more selective structures for HCHO synthesis from DME.