The one-step selective synthesis of dimethoxymethane (DMM; CH3OCH2OCH3) was achieved by oxidation of dimethyl ether (DME) or methanol (CH3OH) with O2 at low temperatures (453−513 K) on unsupported and SiO2-supported heteropolyacids with Keggin structures [H3+nPVnMo12-nO40 (n = 0−4)]. These materials provide redox and Bronsted acid sites required for bifunctional DMM synthesis pathways. Supported structures at submonolayer coverages (0.1−0.28 Keggin units per nm2) are much more accessible than bulk structures and remove diffusional constraints. Their higher dispersions lead to marked improvements in DMM synthesis rates and selectivities and to lower COx yields using either CH3OH or DME reactants. The presence of H2O during DME oxidation increases DMM synthesis rates because of a consequent increase in the rate of DME hydrolysis reactions, which form CH3OH molecules required as intermediates in the DMM synthesis reaction sequence. Pure CH3OH reactants form DMM at much higher rates than DME reactants. The re...