6-[18F]Fluoro-L-dopa and 6-[18F]fluorodopamine are promising PET imaging agents for visualizing cerebral dopaminergic centers and cardiac sympathetic innervation and function. Administration to humans requires a means to determine the purity before injection. We describe such a method using HPLC with u.v. and radioactivity detection and a single high-speed C-18 column with gradient elution. The procedure can resolve within 10 min these fluorinated catechols, their isomers, and dihydroxyphenylalanine. The chemical and radiochemical purity, and specific activity, can be determined before injection.