Adsorption of carbon dioxide and other gases on a novel acetate functionalized silica adsorbent under various conditions is investigated experimentally and theoretically by grand-canonical Monte Carlo (GCMC) simulations. The acetate functional group has the capability to interact with the carbon atoms in CO2 molecules due to the electron-donating properties of the carbonyl and ether groups. However, the acetate functional group has not yet been examined for CO2 adsorption on silica. Adsorption of CO2, CH4, N2, and H2 was measured experimentally in the temperature range of 253–373 K and pressure range of 0–100 kPa. CO2 showed significantly higher adsorption compared to other gases with maximum adsorption of ca. 32 cc/gr at standard condition (STP) at a pressure of 100 kPa and a temperature of 253 K. The recorded adsorption data could be fitted by Freundlich isotherms, indicating heterogeneous adsorption sites. To better understand the heterogeneous adsorption sites, GCMC simulations were used to examine the effects of pore size, temperature, pressure, concentration of functional groups in the silica matrix, and competitive adsorption. The GCMC data was in good agreement with the experimental data and suggested the oxygen-containing moieties (i.e., carbonyl and ether groups) on the acetate group as the adsorption sites. These sites displayed high Lewis acid-base interaction with the CO2 molecules. The GCMC data indicated selective adsorption of CO2 over N2 and a CO2/N2 binary gas mixture selectivity of 20 for a 10/90 CO2/N2 feed. To the best of our knowledge, this is the first report on the experimental adsorption of CO2 over acetate functionalized silica adsorbent coupled with an investigation of the adsorption sites through GCMC simulations.