The F-18-labeling of 5-(trifluoromethyl)dibenzothiophenium trifluoromethanesulfonate, commonly referred to as the Umemoto reagent, has been accomplished applying a halogen exchange F-18-fluorination with F-18-fluoride, followed by oxidative cyclization with Oxone and trifluoromethanesulfonic anhydride. This new F-18-reagent allows for the direct chemoselective F-18-labeling of unmodified peptides at the thiol cysteine residue.