The long term stability of process solutions used in H.sub.2 S-to-sulfur absorption/oxidation process, wherein the incoming hydrogen sulfide-containing gas stream is contaminated with hydrocarbons and/or organic oxygenated compounds and the process is generating an excessive amount of thiosulfate in the process solution, is improved by a method comprising contacting the gas stream and/or the process solution with charcoal to remove both the contamination and at least a portion of the thiosulfate therefrom.