Abstract: Process for working up the scrubbing solution obtained during the scrubbing of SO.sub.2 -containing waste gases, which can also contain SO.sub.3, with the use of ammonia, by evaporation in direct heat exchange with hot cracked gas to a water content of between 15 and 55% by weight, cracking of the thus-evaporated mixture at temperatures of between 900.degree. and 1250.degree. C. with the concomitant combustion of a carbon- and/or sulfur-containing fuel to an SO.sub.2 -containing gas, and processing of the cracked gas, after utilization for evaporation purposes, by contact oxidation to sulfuric acid in accordance with U.S. patent application Ser. 284,709 filed Aug. 30, 1972, characterized in that the scrubbing solution is mixed, prior to or during the evaporation, with a quantity of sulfuric acid sufficient for the liberation of SO.sub.2 from 0-83% of the sulfite and/or bisulfite present in the solution.