In a contacting zone a sour gaseous stream is contacted at a temperature below the melting point of sulphur with an aqueous reactant solution, which reactant solution contains an ammonium form of a coordination complex of Fe(III) with nitrilotriacetic acid, an ammonium form of a coordination complex of Fe(II) with nitrilotriacetic acid, and aqueous ammonia, and which reactant solution has a pH of between about 5 and about 8.5, under conditions to convert H2S, and producing a gaseous stream having reduced H2S content and an aqueous mixture containing solid sulphur and additional coordination complex of Fe(II) in solution, wherein the molar ratio of the coordination complex of Fe(III) to the coordination complex of Fe(II) is between about 0.2 and about 6, and wherein the aqueous reactant solution further is selected from the molar ratio of an amount of Fe to the amount of thiosulphate ion of about 0.01 to about 4. |