1 - arylalkoxy - trisphosphonium salts of the formula i ## str1 ## wherein the substituents r 1 to r 5 are equal or different and represent hydrogen , alkyl having from 1 to 6 carbon atoms which may be perfluorinated , halogen , alkoxy or alkylthio , each having from 1 to 6 carbon atoms , y represents hydrogen or a perfluoroalkyl group c n f 2n + 1 having from 1 to 6 carbon atoms , x represents bromine or iodine and &# 34 ; alkyl &# 34 ; represents an alkyl group of 1 to 3 carbon atoms , at most 3 , preferably at most 2 , of the groups r 1 to r 5 having a meaning other than hydrogen and the alkyl , alkoxy and alkylthio substituents attached to the aromatic nucleus altogether containing preferably at most 6 carbon atoms , in particular at most 4 carbon atoms . the invention also relates to a process for the preparation of the afore - mentioned phosphonium salts and to the use of the compounds of the formula i for the preparation of aromatic compounds having partly fluorinated groups of the formula v ## str2 ## in which r 1 to r 5 , x and y have the afore - mentioned meaning , x can however also be hydrogen .