In a method for operating a flue gas purification plant (10) including a plurality of parallel of absorber chambers (11), in which in each absorber chamber (11), CO and NO are simultaneously oxidized by a catalyst in a first absorber (15) according to the SCONOx principle and the resulting NO.sub.2 is absorbed on the catalyst surface, in which SO.sub.2 is furthermore oxidized by a catalyst in a second absorber (14) upstream of the first absorber (15) according to the SCOSOx principle and the resulting SO.sub.3 is absorbed on the catalyst surface. The absorber chambers (11) are successively regenerated by a regeneration gas containing hydrogen and/or hydrogen compounds in regularly repeating regeneration cycles affecting all the absorber chambers (11). In order to improve reliability and reduce operating costs in such a method, the regeneration time of the second absorber (14) within the regeneration cycle is respectively selected to be long enough to guarantee sufficient regeneration of the second absorber (14).