Treatment of 1,3,5-trimethoxybenzene with one molar equivalence of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA-BF 4 ) under mild conditions (-40 to +20 o C) in acetonitrile gave mainly 2-fluoro-1,3,5-trimethoxybenzene (62% yield; all product yields quoted refer to isolated pure materials, unless otherwise stated) and some 4,4-difluoro-3,5-dimethoxycyclohexa-2,5-dien-1-one (13% yield), the structural parameters of which were determined by X-ray crystallography. Evidence that the minor product arose via fluorodemethylation of the 2-fluoro-1,3,5-trimethoxybenzene formed first was obtained in a separate experiment involving this fluoroaromatic compound and F-TEDA-BF 4 as the sole reactants. Treatment of 1,3,5-trimethoxybenzene with a two molar equivalence of F-tEDA-BF 4 in acetonitrile (at -20 to +20 o C) produced only 4,4-difluoro-3,5-dimethoxycyclohexa-2,5-dien-1-one (>50% yield),a conversion which was unaffected by carrying out the reaction in either aqueous acetonitrile (20 vol% H 2 O) or neat water. Treatment of 2,4,6-trimethoxytoluene with a two-molar equivalence of F-TEDA-BF 4 in acetonitrile between -20 and +20 o C gave the fluorodemethylation product 4-fluoro-3,5-dimethoxy-4-methylcyclohexa-2,5-dien-1-one in 81% yield.