Sulfur dioxide (SO2) may be removed from carbon dioxide feed gas by contacting the carbon dioxide at an elevated temperature and an elevated pressure with a catalyst for oxidizing SO2, in the presence of oxygen (O2) to convert SO2 to sulfur trioxide (SO3); contacting SO3 in the resultant SO3-enriched carbon dioxide gas with water to produce sulfuric acid and SO2-depleted carbon dioxide gas; and separating the sulfuric acid from the SO2-depleted carbon dioxide gas. If present, NOx is also removed from the carbon dioxide feed gas as nitric acid to produce SO2-depleted, NOx-lean carbon dioxide gas. The method has particular application in the removal of SO2 and NOx from flue gas produced by oxyfuel combustion of a hydrocarbon fuel or carbonaceous fuel, within or downstream of the CO2 compression train of a CO2 recovery and purification system.