Selective functionalization of methane at moderate temperature is of crucial economic, environmental and scientific importance. Here we report that methane reacts with heteropolyacids chemisorbed on silica to produce acetic acid under soft conditions. Specially, when chemisorbed on silica, H4SiW12O40, H3PW12O40, H4SiMo12O40 and H3PMo12O40 activate the primary C-H bond of methane at room temperature and atmospheric pressure. With these systems, acetic acid is produced directly from methane, in a single step, in the absence of Pd and without adding CO. Extensive surface characterization by solid-state nuclear magnetic resonance (NMR) spectroscopy, infrared spectroscopy, cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) suggests that C-H activation of methane is triggered by the protons in the HPA-silica interface with concerted reduction of the Keggin cage, leading to water formation and hydration of the interface. This is the simplest and mildest way reported to date to functionalize methane.