The invention relates to a device and a process for nucleophilic fluorination of a substance, especially for synthesis of an .sup.18F-labeled substance for examination using a positron emission tomograph. The device comprises an anion exchange device (102) for extraction of [.sup.18F]fluoride ions by means of adsorption from a target fluid, whereby the anion exchange device (102) can be charged via a supply device (101) with the target fluid; and a measurement device (104) with a measurement chamber (103) for measuring initial radioactivity of the [.sup.18F]fluoride ions, the anion exchange device (102) being arranged at least partially in the measurement chamber (103) of the measurement device (104).