The present invention provides a phosphonium compound of formula (II). Also provided is a method for producing a quaternary phosphonium compound labeled with a positron emitting radionuclide, the method comprising the step of reacting an electrophile of formula (I): X 1 -CH 2 -A 1 with triphenylphosphine having one or more radionuclide-labeled substituents on the benzene ring to give a quaternary phosphonium salt.
