Controlling the reductive capacity of an aqueous alkaline slurry (23) in a wet scrubber makes it possible to accurately control the mercury emission from the scrubber to a desired value. One method of controlling the reductive capacity of the slurry is to measure the reduction-oxidation potential ("redox potential") of the aqueous alkaline slurry (23) and to add or remove substances that affect the redox potential and thus the reductive capacity of the slurry. In wet scrubbers in which limestone is used for absorption of acid gases and where a gypsum slurry is circulated, it has been found to be an attractive solution to control the amount of oxidation air blown into the scrubber in order to control the redox potential and thereby the mercury emissions.