The acidic oxide SO2 and protic acid HCl are among the gases released in the combustion of coal and the incineration of municipal waste. They are typically removed by wet or dry scrubbing involving calcium carbonate or calcium hydroxide. The molten alkali carbonate eutectic provides a liquid-state alternative that readily absorbs SO2 and HCl and does not become covered with a passivating layer. Gas-liquid scattering experiments utilizing the eutectic mixture (44 mol % Li2CO3, 31 mol % Na2CO3, 25 mol % K2CO3) reveal that the reaction probability for HCl(g) + CO32- --> CO2(g) + OH- + Cl- is 0.31 +/- 0.02 at 683 K and rises to 0.39 at 783 K. Gaseous CO2 is formed within 10(-4) s or less, implying that the reaction takes place in a liquid depth of less than 1000 angstrom. When the melt is exposed to D2O, the analogous reaction D2O(g) + CO32- --> CO2 (g) + 2OD(-) occurs too slowly to measure and no water uptake is observed. Together with previous studies of SO2(g) + CO32- --> CO2(g) + SO32- these results demonstrate that molten carbonates efficiently remove both gaseous HCl and SO2 while reacting at most weakly with water vapor. The experiments further highlight the remarkable ability of hot CO32- ions to behave as a base in reactions with protic and Lewis acids.