An injector mixer arrangement (10) for supplying a reducing agent in gaseous form into a flue gas flowing in a gas duct (14) communicating with a catalyst (18a) in a selective catalytic reduction (SCR) reactor (12) arranged downstream of said injector mixer arrangement (10). The injector mixer arrangement (10) comprises an injector grid (22) equipped with a plurality of nozzles (30) arranged horizontally within the gas duct (14). The nozzles (30) are adapted to supply said reducing agent to the gas duct (14). The injector mixer arrangement (10) further comprises first stage mixer plates (24) and second stage mixer plates (26) arranged in the gas duct (14) downstream of said nozzles (30) and upstream of SCR reactor 12.