Acetic acid (AA) is an important bulk commodity chemical produced primarily via methanol carbonylation using homogeneous organometallic catalysts in liquid phase reactors with halides that participate in each catalytic turnover. Here we report a heterogeneous, halide free, gas-phase process for methanol carbonylation to AA using a catalyst consisting of atomically dispersed rhodium (Rh) active sites on an acidic support. It is demonstrated that active site pairs consisting of atomically dispersed Rh and support acid sites enable highly selective AA production, whereas Rh clusters drive methanol decomposition to CO and CO,. Through a comparison of methanol carbonylation over atomically dispersed Rh species on gamma-Al2O3, ZrO2, and Na-modified ZrO2, it was identified that by decreasing the concentration of acidic sites, the production of byproduct dimethyl ether could be minimized. After the acidity of the support was tuned by depositing 5 wt % Na on ZrO2 that contained atomically dispersed Rh, AA selectivity was promoted to 54% in a stable-gas phase, halide-free process operating at a stoichiometric methanol to CO feed ratio. Mechanistic analysis suggested that AA formation proceeded through a bifunctional reaction mechanism where CO adsorbed to Rh inserted into methoxy species on adjacent acid sites in the kinetically relevant step. This work illustrates the role of Rh structure and support acidic sites on methanol carbonylation and provides insights into how paired sites of atomically dispersed metals and acidic sites can be manipulated to drive important catalytic processes.