A process for the prepn. of monofluoromethylated org. biol. active compds. using monofluoromethylated reagents was disclosed. Thus, fluticasone propionate and fluticasone furoate I [R = CH2Me, 2-furanyl, resp.] were prepd. using S-monofluoromethyl-S-phenyl-2,3,4,5-tetramethylphenylsulfonium tetrafluoroborate as monofluoromethylating reagent instead of bromofluoromethane.