The carbonate radical anion CO3 center dot- is a key intermediate in tropospheric anion chemistry. Despite its radical character, only a small number of reactions have been reported in the literature. Here we investigate the gas-phase reactions of CO3 center dot- and CO3 center dot-(H2O) with HCl under ultrahigh vacuum conditions. Bare CO3 center dot- forms OHCl center dot- with a rate constant of 4.2 x 10(-12) cm(3) s(-1), which corresponds to an efficiency of only 0.4%. Hydration accelerates the reaction, and ligand exchange of H2O against HCl proceeds with a rate of 2.7 X 10(-10) cm(3) s(-1). Quantum chemical calculations reveal that OHCl center dot- is best described as an OH center dot hydrogen bonded to Cl-, while the ligand exchange product is Cl-(HCO3 center dot). Under tropospheric conditions, where CO3 center dot-(H2O) is the dominant species, Cl-(HCO3 center dot) is efficiently formed. These reactions must be included in models of tropospheric anion chemistry.