This invention relates to novel fluorinated phenyl thiphenyl (also named diarylsulfide) derivatives and their use in Positron Emission Tomagraphy (PET) imaging of Serotonin Transporters (SERTS). The present invention also provides diagnostic compositions comprising the novel compounds of the present invention, and a pharmaceutically acceptable carrier or diluent. The invention further provides a method of imaging SERTS, comprising introducing into a patient a detectable quantity of a labeled compound of the present invetion, or a pharmaceutically acceptable salt, ester, amide or prodrug thereof.