Aryliodonium precursors are widely applied for copper-free labeling of positron emission tomography (PET) tracers with fluorine-18. We assessed 18F-fluoroarene regioisomer formation in examples of these labeling methods. Aryliodonium ylides derived from Meldrum's acid bearing para electron-donating groups react with [18F]fluoride in acetonitrile to produce regioisomeric 18F-fluoroarenes via a competing aryne pathway. Regioisomer formation is decreased or absent in DMF. Analytically checking for the absence of the 18F-regioisomer from any particular PET tracer radiosynthesis using these or similar methods is recommended.