Abstract: A process for removing sulfur dioxide and other contaminants from a hot waste gas stream in which the waste gas stream is cooled to a temperature below 100.degree.F and then passed in sequence through two separate absorption zones utilizing as the absorbent a recirculated aqueous solution of ammonium sulfite-bisulfite salts. Heat is continuously withdrawn from the recirculated salt solution in each zone. The absorbent solution in the first zone has an initial pH of from 6 to 7.2 and a salt concentration of from 1 to 20% by weight. The absorbent solution in the second zone has an initial pH of from 5.2 to 6.5 and a salt concentration, lower than said first zone of, from 0.1 to 5% by weight. The process provides for the efficient absorption and substantial removal of SO.sub.2, SO.sub.3 and particulate contaminants and for the economic recovery and use of heat from stack gases of a variety of industrial and commercial operations.