A powdered Iron-Selenide-Oxide sorbent composition, a method of using the sorbent, and a method for producing the sorbent is disclosed for removing mercury vapor from a gaseous stream. The exact ratios of Fe to Se are varied. The method for producing the sorbent comprises the steps of: preparing a base ferrous or ferric oxide (FeOx) media, conditioning the FeOx media with a slurry of ferrous selenite (FeSeO3)/ferrous selenate (FeSeO4), reducing the FeSeO3/FeSeO4 mixture by an elemental iron (Fe0) powder to incorporate selenide into the slurry, and dewatering and oxidizing the slurry. The sorbent can be used alone (or in combination with a powdered activated carbon material or an inert powdered material) by direct injection into the Hg-containing gas stream, by coating of a filter fabric used in bag house with the sorbent, or by using the sorbent in a packed bed gas stream filtration system.