Abstract: A process for the removal of sulfur dioxide and other contaminants and the recovery of ammonium sulfate as a by-product from a hot waste gas stream comprising cooling the waste gas stream to a temperature below 120.degree. F., passing said waste gas stream in sequence through at least two separate absorption zones utilizing as an absorbent a recirculated aqueous salt solution of ammonium sulfite-bisulfite, withdrawing and oxidizing a portion of said aqueous ammonium sulfite-bisulfite salt solution under pressures and temperatures sufficient to substantially convert the salts to sulfate salts, and recycling the ammonia and sulfur dioxide vapors liberated during oxidation to the first absorption zone.