An improvement is provided in the regeneration system of agueous regenerative processes for the removal of SO.sub.2 from SO.sub.2 -containing gas streams which have a scrubbing system containing (1) a scrubbing zone through which a recirculating stream of sodium, potassium or ammonium thiosulfate solution continuously passes as a vehicle for the SO.sub.2 absorbent--sodium, potassium, or ammonium carbonate--under conditions favorable to the formation of sulfite by the reaction of carbonate and SO.sub.2 and (2) a thiosulfate generation zone in which the sulfite is converted by sulfide to thiosulfate. The improvement comprises converting in a regenerating system, the net make of thiosulfate to sulfide and carbonate by converting one portion of the thiosulfate to formate and the reacting the other portion with said formate.