Mercaptans (RSH) occur naturally in crude oils and are also generated from other sulfur compounds during crude fractionation and cracking processes. Mercaptans are undesirable in gasoline because of their obnoxious odor and their tendency to hydrolyze, forming toxic and corrosive hydrogen sulfide. The classic tests for mercaptan presence are the "doctor" test and odor threshold.

The Axens Sulfrex^TM and Sweetening processes eliminate mercaptans by extraction or by their conversion into less aggressive compounds protecting downstream equipment or units such as hydrotreaters as well as meeting fuel specifications.

The total licensed capacity for twenty-two Sulfrex^TM and Sweetening units, exceeds 22 500 tons per day.