Hydrogen sulfide is oxidized to sulfur by means of treatment with an aqueous acid solution containing trivalent iron and a hetero polyacid having formula (I): H.sub.nXV.sub.yM.sub.(12-y)O.sub.40; or a sole hetero polyacid having formula (II): H.sub.nMeM.sub.12O.sub.40; wherein the symbols X, M Me n and y are specified in the text. At the end of the oxidation, the solution is treated with gas containing oxygen to re-oxidize the reduced metal.