Readily available ascorbic acid was discovered as an environmentally benign hydrogen bond donor for the synthesis of cyclic organic carbonates from CO2 and epoxides in the presence of nucleophilic cocatalysts. The ascorbic acid/TBAI (TBAI: tetrabutylammonium iodide) binary system could be applied for the cycloaddition of CO2 to various epoxides under ambient or mild conditions. Density functional theory calculations and catalysis experiments revealed an intriguing bifunctional mechanism in the step of CO2 insertion involving different hydroxyl moieties (enediol, ethyldiol) of the ascorbic acid scaffold.