In a contacting zone (2) a sour gaseous stream (1) is contacted at a temperature below the melting point of sulphur with an aqueous reactant solution (12), 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 ammonium form of the coordination complex of Fe(II) with nitrilotriacetic acid in solution, wherein the molar ratio of the ammonium form of the coordination complex of Fe(III) with nitrilotriacetic acid to the ammonium form of the coordination complex of Fe(II) with nitrilotriacetic acid in the reactant solution is between about 0.2 and about 6, and wherein the aqueous reactant solution optionally comprises 0.01 mol to 4 mol thio­sulphate ion per mol of iron.