The invention relates to a process for producing a compound comprising the anion [CF
2
18
FSO
2
]−, which process comprises treating a difluorocarbene source with (i) a source of
18
F
−
and (ii) a source of SO
2
. The invention relates to a compound which comprises that anion. The invention also relates to the use of the compound comprising the anion [CF
2
18
FSO
2
]
−
to produce a compound comprising an
18
F-trifluormethyl functionalised aromatic group. Compounds comprising an
18
F-trifluoromethyl functionalised aromatic group are also the subject of the present invention.