Sulfur dioxide (SO 2 ) is removed from a carbon dioxide feed gas by maintaining the feed gas at elevated pressure(s) in the presence of oxygen (O 2 ), water and NO x for a period of time sufficient to convert SO 2 to sulfuric acid and NO x to nitric acid and produce SO 2 -depleted, NO x -lean carbon dioxide gas. The invention resides in separating the sulfuric and nitric acids from said SO 2 -depleted, NO x -lean carbon dioxide gas, and then neutralizing the acids by reaction with an alkaline sorbent in an acid/sorbent reactor system to produce sorbent-derive sulfate and nitrate. The method has particular application in the removal of SO 2 and NO x from flue gas produced by oxyfuel combustion of a carbonaceous fuel.