One aspect of the present invention relates to a method of preparing radiofluorinated substituted alkyl, cycloalkyl, aryl, and alkenyl compounds. In a preferred embodiment, potassium fluoride-18 is used. Another aspect of the invention relates to arylammonium compounds containing fluorine-18 that are useful as imaging agents. In certain embodiments, the ammonium compound is a tetraaryl ammonium salt. Another aspect of the invention relates to arylsulfonium compounds containing fluorine-18 that are useful as imaging agents. In certain embodiments, the sulfonium compound is a triaryl sulfonium salt. Another aspect of the present invention relates to a method of obtaining a positron emission image of a mammal, comprising the steps of administering to a mammal a compound of the invention, and acquiring a positron emission spectrum of the mammal.