A process for removing contaminants from a waste gas stream comprises treating the waste gas stream to remove at least one of SO.sub.2 and NO.sub.x and to obtain a lean stream having a reduced level of at least one of SO.sub.2 and NO.sub.x and, contacting the lean gas stream with a mercury absorbent solution comprising permanganate to remove mercury vapour and to obtain a mercury lean stream and a mercury rich absorbent solution. The mercury rich absorbent solution may be subsequently treated on a batch basis to remove precipitated manganese dioxide and obtain a solution containing mercury ions.